Sport Guides
Ironman 70.3 Training with Garmin: A Complete Data-Driven Guide
April 16, 2026
Why 70.3 training is different from single-sport training
A 70.3 is not a hard swim followed by a hard bike followed by a hard run. It is one continuous physiological event. How you swim affects how you bike. How you bike determines everything about the run. That chain of consequences is what makes multi-sport training structurally different from preparing for a marathon or a century ride.
Most age-groupers finish a 70.3 in 5–7 hours. That is a long time to manage effort across three disciplines, with a transition sandwich in the middle. The aerobic demand is relentless, but the intensity stays moderate — which means your pacing discipline and your accumulated fatigue going into race day matter far more than your single-session peak fitness.
The core training challenge is total weekly load management across three sports. You cannot just stack your previous marathon training and add swimming and cycling on top. The load adds up faster than you expect, and the cumulative fatigue from three sports hits differently than single-sport volume. A 60-minute tempo run the day after a 3-hour ride is a very different physiological stimulus than that same run on fresh legs.
Recovery between disciplines also matters more here. A hard swim set on Tuesday will affect your FTP interval session on Wednesday. That interdependency is invisible if you are planning each sport in isolation. When you are only looking at your running training load or your cycling training load, you are missing the total picture.
And then there is the bike-to-run relationship. Go too hard on the bike — even by 5–10 watts above your target — and your run will fall apart. The number of 70.3 athletes who have a strong swim and bike and then walk the back half of the run is very high. That relationship is where races are won and lost, and it has to be trained specifically.
The metrics that matter across all three sports
Before you start logging miles, you need a clear picture of what you are actually measuring and why.
Combined training load is the single most important metric to track. Garmin calculates training load per activity, but for 70.3 training you need to watch the aggregate across all three sports every week. A training load ratio above 1.5 is a red flag — it means you are accumulating stress faster than your body can adapt to it. Keep that ratio between 0.8 and 1.3 for sustainable progression.
FTP (functional threshold power) is the foundation of your cycling training and your race-day pacing strategy. If your FTP is wrong, your target power for the 90 km bike leg is wrong, and your run will suffer the consequences. Test it properly before you start the block and retest at week 10 — estimate your FTP and get all 7 power zones here. See our FTP testing guide for the protocols that work best with Garmin.
Lactate threshold pace for running is the equivalent on the run side. Garmin estimates this from your race and workout data. Use it to set your threshold run targets and your race-day run pace ceiling. Do not run to feel on a 70.3 — run to data, at least for the first 15 km.
CSS (critical swim speed) is the swim equivalent of FTP. It represents the fastest pace you can theoretically sustain indefinitely — in practice, it is your race-pace swim target. Garmin does not calculate CSS natively, but you can test it in a pool: swim a 400 m time trial and a 200 m time trial in the same session. CSS = (400m distance − 200m distance) ÷ (400m time − 200m time). Set this as your interval target for CSS sets.
HRV and training readiness are where Garmin earns its keep for multi-sport athletes. HRV reflects accumulated stress across all three sports — your nervous system does not separate swim fatigue from run fatigue. If your readiness score is low on a planned threshold bike day, that is real information. Use it. See how to set up heart rate zones correctly first, because the rest of the readiness data flows from accurate zone calibration.
Body Battery is particularly useful in multi-sport training because it captures the cumulative drain of training twice a day or on back-to-back days. A Body Battery below 40 going into a key session is worth paying attention to. You do not need to skip the session, but you should adjust the expectations.
20-week training structure
The block is built in four phases. Each phase has a different emphasis, but the logic is the same throughout: accumulate load, absorb it, repeat at a higher level.
| Phase | Weeks | Focus | Swim (hrs) | Bike (hrs) | Run (hrs) | Total (hrs) |
|---|---|---|---|---|---|---|
| Base | 1–8 | Aerobic volume, technique | 2–3 | 5–7 | 3–5 | 10–15 |
| Build | 9–14 | Threshold work, bricks | 2–3 | 6–9 | 4–6 | 12–18 |
| Peak | 15–18 | Race-specific, long bricks | 2–3 | 7–10 | 4–6 | 13–19 |
| Taper | 19–20 | Freshness, sharpening | 1–2 | 3–5 | 2–3 | 6–10 |
Weeks 1–8: Base
The base phase is about building the aerobic engine in all three sports simultaneously. Run volume and cycling volume go up gradually — no more than 10% per week on either. Swimming in this phase is mostly technique and aerobic development: longer sets at comfortable pace, open-water practice if your race is in open water.
This is also when you establish your benchmarks. Test your FTP in week 2. Establish your CSS. Run a lactate threshold test. These numbers are your training anchors for the entire block.
Every third or fourth week should be a recovery week — drop volume by 30–40% and let the training land. Zone 2 training dominates this phase. Keep intensity disciplined. Resist the urge to push hard in base — you will pay for it in the build phase.
Weeks 9–14: Build
This is the hardest phase of the block. You are now adding threshold-level work in all three sports while maintaining the aerobic base. Brick sessions appear here for the first time: typically a ride followed immediately by a run, starting short (45 min ride + 15 min run) and building to longer efforts by week 14.
Cycling gets the most structured intensity in this phase. FTP intervals (2×20 min at threshold, or 4×10 min) are the core bike session. Run lactate threshold intervals (4×1 mile at threshold pace, or cruise intervals) appear weekly. Swim moves toward CSS intervals and race-pace continuous sets.
Watch your training load ratio closely in weeks 11–13 when load is highest. This is when overtraining risk peaks.
Weeks 15–18: Peak
Peak phase is race-specific. The key sessions are long bricks — 2.5–3 hour rides at race power, immediately followed by 30–45 minute runs at race pace. These sessions are physically hard but they also train the mental and pacing skills you need on race day.
Volume is slightly lower than the build peak but intensity at race specificity is high. Long rides include 60–90 minutes at race power (intensity factor 0.76–0.82). Long runs include race-pace miles in the final third. Swimming focuses on race-pace continuous sets and open-water sighting.
Weeks 19–20: Taper
Volume drops sharply. Intensity stays present but effort is short. The goal is to arrive at race day with empty fatigue and full fitness. See our race taper guide for Garmin athletes for how to read your metrics during these two weeks and avoid the common mistake of tapering too aggressively.
Key workouts by sport
Swim
The three swim sessions that matter most are CSS intervals, open-water sighting practice, and race-pace continuous sets.
CSS intervals: 8–12 × 100 m at CSS pace with 15 seconds rest. These build sustainable race-pace swimming and improve your aerobic threshold in the water. Add CSS sets as the block progresses — 6 × 200 m at CSS by peak phase.
Open-water sighting: if your race is in open water, practice sighting every 6–10 strokes. Sighting costs time and energy. Train it until it is automatic.
Race-pace continuous sets: in peak phase, swim 1,500–1,900 m continuously at race pace. This is your dress rehearsal for the swim leg. It also shows you what your heart rate looks like post-swim — useful for calibrating your early bike effort.
Bike
The three key bike sessions are FTP intervals, tempo rides, and long rides with race-pace segments.
FTP intervals are the engine of cycling fitness. 2×20 min at FTP, or 3×15 min at 95–100% FTP, with 5-minute recoveries. These should be structured workouts on Garmin with power targets — see our guide to Garmin cycling structured workouts for how to build and execute them.
Tempo rides (2–3 hours at 75–85% FTP) build the aerobic base that keeps you fresh through a long bike leg. These feel moderate but accumulate a lot of beneficial training stress.
Long rides in peak phase: 3–4 hours total, with 60–90 minutes at race power (your target IF on race day). These teach your body to sustain race effort for a prolonged period and give you the pacing data you need.
Run
Threshold intervals, brick runs, and long runs.
Threshold intervals: 4–6 × 1 mile at lactate threshold pace, or 20–30 minutes of cruise intervals at threshold. These are the primary run fitness driver in the build and peak phases.
Brick runs are the most important run training in the block. Getting off the bike and running immediately is a skill — your legs feel strange, your pace feels wrong, your HR is elevated. The goal of brick runs is to shrink the transition adaptation time. After 8–10 brick sessions, the first kilometre off the bike stops costing you.
Long runs cap at 1:45–2:00 in peak phase. A 70.3 run leg is 21 km. You do not need to run 21 km in training — the bike before it effectively pre-loads your legs. Train to 75–80% of the race run distance.
How to read your Garmin data through the block
Build a weekly review habit. Every Sunday, look at four things: total training load across all three sports, your HRV trend over the past 7 days, your readiness score trend, and your VO2 max estimates for bike and run.
The load picture tells you whether the week was appropriate. The HRV and readiness trends tell you whether your body absorbed it. VO2 max trends tell you whether your fitness is actually progressing.
Red flags to act on:
A training load ratio above 1.5 means reduce load this week regardless of how you feel. This metric precedes overtraining — act before symptoms appear.
HRV dropping for three or more consecutive weeks is a sign of accumulated stress that is outpacing recovery. Insert a recovery week. Do not try to push through it.
Flat or declining VO2 max in weeks 10 and beyond usually means intensity is too low or you are under-recovered. If load is appropriate and HRV is stable, the fix is more intensity. If HRV is suppressed, the fix is more recovery first.
Brick session data deserves its own analysis. After each brick, look at the relationship between your bike average power and your first-kilometre run HR and pace. Over the training block, you should see your run HR drop at the same post-bike pace — that is the physiological adaptation you are looking for. If your run HR is still spiking hard in week 16 after a race-pace bike, your pacing strategy or fitness needs adjustment.
The training load ratio guide and Garmin triathlon training overview go deeper on how to interpret these metrics week to week.
Race-day execution
Swim: race effort, not race pace. Start conservatively. In open water with a mass or rolling start, the first 200–400 metres will be chaotic. Find your rhythm before you find your pace. Going 10 seconds per 100 m too hard in the swim costs you far more on the bike than those seconds gained.
Bike: this is where your FTP work pays off. Target an intensity factor of 0.76–0.82 for athletes aiming for sub-5 hour total times. Longer race times warrant lower IF — 0.72–0.76 for 5.5–7 hour finishes. Set power targets on your Garmin before the race and ride to them. Ignore the athletes passing you in the first 30 km. Stick to your number.
Set up your Garmin data fields before race day: 3-second average power, normalized power, lap pace, lap HR, and elapsed time are the essentials. Nutrition alerts are worth adding if you use them — a prompt every 20–25 minutes keeps fuelling consistent.
Run: HR-controlled for the first 5 km, not pace. Your pace will feel slow. Run it anyway. The bike has pre-fatigued your legs and elevated your baseline HR. Runners who go by feel in the first 5 km of the 70.3 run are the ones walking by kilometre 15. Lock in your HR ceiling for that first quarter of the run, then gradually allow pace to come down if HR stays controlled. You can predict your run-leg finish time from a recent standalone half marathon using the Riegel formula to set a realistic pace target.
The multi-sport coaching gap
Garmin Coach offers guided plans for running and cycling, but it does not have a 70.3 training plan. That is not a minor gap — a 20-week multi-sport block with three disciplines, multiple load streams, and interdependent recovery is a genuinely complex planning challenge.
TrainingPeaks has 70.3 plans from well-known coaches, but those plans are static. They do not know that your HRV dropped this week, that your running training load has been trending high, or that your long brick last weekend left you more fatigued than expected. They cannot shift Thursday's run to Saturday because your readiness is low.
A multi-sport AI coach that reads your actual Garmin data — load across all three sports, HRV, readiness, body battery — and adjusts training week by week is a different category of tool. That is what Gneta does. It reads your training load and recovery data, coordinates the three sport streams, and surfaces the weekly insights that help you train smarter across a 20-week block.
If you are building toward a 70.3, explore the Gneta features or see how it compares to TrainingPeaks. The pricing is $7.99/month — less than a single coaching session.
Related reading: AI triathlon coach overview · Garmin triathlon training guide · FTP testing guide · Race taper with Garmin · Zone 2 training on Garmin
