The Half Ironman (officially branded as IRONMAN 70.3) is the fastest-growing format in triathlon, combining a 1.9km open-water swim, 90km bike, and 21.1km run. Finishing times range from 3:50 (elite) to 8:30+ (age-group back-of-pack), with average age-group finishers completing the course in approximately 5:30–6:30. The 70.3 sits at a sweet spot: demanding enough to require structured multi-month preparation but achievable for athletes with 8–12 hours per week to train.
Physiological Demands and Key Limiters
At race pace, a 70.3 is performed primarily at upper Zone 2 to lower Zone 3 intensity (75–85% of maximum heart rate, roughly 65–75% of FTP on the bike). The primary physiological limiter for most age-group athletes is aerobic capacity — the ability to sustain high metabolic output for 4–7 hours. Secondary limiters include: fueling capacity (gut tolerance for carbohydrates at race intensity), swim efficiency (poor technique exponentially increases energy cost), and bike-to-run fitness (brick-specific adaptation takes months to develop).
Most athletes underinvest in swimming and overinvest in running. For the average triathlete, improving swim efficiency from 2:00/100m to 1:45/100m saves 6–7 minutes on race day with minimal additional cardiovascular cost — a highly efficient time investment.
20-Week Periodization Structure
The 20 weeks divide into four phases:
- Weeks 1–5 (Base): Aerobic foundation building. Focus on technique in swim, Zone 2 rides 90–120 minutes, easy runs building to 45–60 minutes. Training volume 7–9 hours/week. One brick workout per week (bike then immediate run, 30 min each).
- Weeks 6–12 (Build 1): Introduce race-specific intensity. Swim includes threshold sets. Bike includes 2 × 20-minute threshold intervals. Long bike extends to 100–120 minutes at race pace. Run includes weekly tempo runs. Volume 9–12 hours/week. 1 deload week in weeks 8–9.
- Weeks 13–17 (Build 2 / Race-Specific): Peak training load. Long bike reaches 2.5–3.0 hours. Long brick: 2.5hr bike + 45-min run. Open water swim practice essential. Volume 11–14 hours/week. 1 deload week in week 15.
- Weeks 18–20 (Taper): Volume reduces 40% then 60%. Maintain intensity in short sharpening sessions. Reduce duration but not frequency. Final race simulation (swim + 45km bike + 10km run) in week 18.
Key Sessions Per Week
In a typical Build 2 week:
- Monday: Rest or easy 30-min swim
- Tuesday: Run — 45 minutes with 20-min tempo at race-pace effort
- Wednesday: Swim — 2,800m with main set 8 × 100m on 1:50, bike — 75 min Z2
- Thursday: Run — 60 min easy with 6 × 1-min strides
- Friday: Swim — 2,200m technique; optional short recovery bike
- Saturday: Brick — 2.5hr bike (last 30 min at race power) + 30-min run at race pace off the bike
- Sunday: Long run — 75–90 minutes at conversational pace
Race-Day Pacing Strategy
The most common 70.3 race-day error: overcooking the bike. Athletes who ride 3–5% above their sustainable power threshold on the bike almost universally pay for it in the final 10km of the run. A conservative bike power target is 70–75% of FTP for well-trained athletes, allowing a strong run. Pace the run slightly conservative for the first 5km — the initial adrenaline surge inflates perceived effort. Target heart rate on the run should be 5–8 bpm below your standalone half-marathon race heart rate.
Fueling the 70.3
Target carbohydrate intake: 60–75g per hour during the bike and run legs. At a 5:30 finish time, that's 3.5–4 hours of fueling after the swim — 210–300g of carbohydrates total. Use a mix of sports drink on the bike (easier to consume) and gels on the run. Practise your exact race nutrition in training on every long bike and brick session — gut training is non-negotiable for 70.3 distances. Hydration targets: 500–750ml per hour on the bike, adjusted for conditions. Begin the run well-hydrated. Use the NorthLine Race Day Nutrition Planner to build your complete 70.3 fueling schedule by expected finish time, and the Sweat Rate Calculator to personalise fluid targets for your target race conditions.