A Half Ironman — 1.9km swim, 90km bike, 21.1km run — is the sweet spot of triathlon. Long enough to demand real preparation, short enough that working athletes can do it without living in the pool or destroying their knees. Most first-timers underestimate how different training for three sports simultaneously is from training for one. This guide gives you the 20-week structure, the key workouts that matter, and the nutrition and taper details that separate finishers from people who blow up on the run.
What a Half Ironman Actually Demands
The 70.3 distances are deceptively tough. The swim is long enough that poor technique costs real time. The bike covers 90km — typically 2h30 to 3h30 for most age-groupers — at an intensity that has to leave your legs functional for the half marathon that follows. The run is where races are won and lost: athletes who bike too hard end up walking the last 7km.
Energy expenditure for a 4h30 70.3 athlete runs to about 3,500–4,000 kcal. Fueling during the race is not optional — it is a skill you train in practice. Total preparation time is typically 8–12 hours per week for 16–20 weeks, peaking at 12–14 hours in the final build phase.
The 20-Week Periodization Structure
A sound 20-week plan divides into four phases:
- Base (weeks 1–6): Aerobic foundation. Easy effort, zone 1–2 work. Weekly TSS builds from ~300 to ~450. Focus: swim technique, cycling efficiency, run durability.
- Build (weeks 7–14): Introduce race-specific intensity. Threshold intervals on the bike, tempo run sessions. TSS climbs to 550–700. This is where fitness is made.
- Peak (weeks 15–17): Highest volume and intensity. One or two race-simulation sessions. TSS reaches 700–800 for your biggest week.
- Taper (weeks 18–20): Volume drops 40–50%, intensity maintained. TSS falls to 350–400 in race week. Body adapts and freshens up.
Use the Training Load Calculator to track your Chronic Training Load (CTL) through each phase.
Weekly Volume and Key Workouts
The workouts that drive adaptation for the 70.3 distance:
- Long ride (weekly): 2h30–4h at zone 2. This builds the aerobic engine for the bike leg. In the build phase, add 20 minutes of sweet spot (88–93% FTP) at the end.
- Threshold bike intervals (weekly): 3–5×8–12 min at FTP. Raises your ceiling so race pace feels controlled.
- Long run (weekly): 70–90 min easy. Never adds the half marathon distance — the race provides that stimulus. Keep it aerobic.
- Swim sets (3×/week): One endurance set (2,500–3,500m), one technique set, one shorter intensity set. Training zone calculator gives you CSS pace.
Brick Training: Running Off the Bike
Running immediately after cycling is the defining skill of triathlon. Your legs have been in a sustained, fixed position for hours; asking them to extend and propel you forward feels profoundly wrong for the first kilometre. Brick workouts train the transition physiology.
Start with short bricks: 45 min bike + 15 min run, every 2 weeks through the base phase. In the build phase, extend to 2h bike + 20–30 min run monthly. One race-simulation brick in peak week: 2h30 ride at race power followed immediately by a 45 min run at race pace.
Target run pace off the bike: start 10–15 sec/km slower than standalone threshold pace. By race day, the gap typically narrows to 5–8 sec/km. Use the Race Predictor to set realistic run split targets.
Nutrition and Fueling
The Half Ironman is too long to race on stored glycogen alone. For a 4h30 finish, you will be racing for over 4 hours, burning roughly 800–900 kcal/hr. Your body can store about 90 minutes of glycogen at race intensity — the remaining 3 hours come from exogenous carbohydrate.
Target: 60–90g of carbohydrate per hour on the bike (start fueling within the first 20 minutes). On the run, gut tolerance drops — aim for 40–60g/hr via gels and cola at aid stations. Sodium: 500–1,000mg/hr depending on sweat rate and temperature.
Gut training is a separate skill. Practice your exact race nutrition during long training rides. The TDEE Macro Planner can calculate your daily calorie and carbohydrate needs through the training block.
The 4-Week Taper
A proper taper feels wrong. Your legs feel flat in week 18, then edgy in week 19, then you start to worry you are losing fitness by race week. This is all normal — it is adaptation. The physiology: you are allowing muscle repair, glycogen supercompensation, and hormonal recovery to complete while maintaining the neuromuscular patterns of fast race effort.
Protocol: cut volume by 40% in week 18, 50% in week 19, 60% in race week. Never cut intensity — keep your interval sessions at race intensity or above, just shorter. One threshold bike workout and one race-pace run per week through the taper. Sleep more, eat more carbohydrate in the final 48 hours.
The Recovery Readiness Calculator tracks your TSB (Training Stress Balance) — you want to arrive at race day with TSB between +5 and +20.
How Adaptive AI Training Changes the Game
A static 20-week plan assumes your life stays constant for five months. It does not account for a work trip in week 11, a minor illness in week 14, or HRV data showing you are under-recovered going into a key build week. Adaptive AI training adjusts the plan in real time.
When your Garmin reports declining HRV, the AI reduces intensity for 48 hours. When you miss a workout, it rebalances the week rather than stacking sessions that lead to injury. When your FTP improves by 15 watts, it recalculates every interval target in the plan. The output is the same structured periodization — just continuously calibrated to your actual state, not a hypothetical average athlete.
A Half Ironman is achievable for most adult athletes who can commit 8–12 hours per week to structured preparation. The plan above gives you the framework. Use the prompt builder below to generate a personalized training week based on your race date, current fitness, and available time.