Long Run Progression for Half Ironman 2026: 12-Week Build to a Strong 21K

A standalone half-marathon starts on fresh legs. The 70.3 half-marathon starts after a 90 km bike ride, low glycogen, dehydration risk, and 3+ hours of accumulated fatigue. Your fastest standalone time means little on race day.

The goal of long-run training isn't running a fast 21K — it's teaching your body to run efficiently on tired legs. Peak long runs of 18–20 km build the durability you need, without the injury risk of chasing full-distance every week.

Every long run at Zone 2 (conversational pace, HR 65–75% of max). Weekly increase capped at 10–15%.

Weeks 1–4 (base): 8 km → 10 → 11 → 12 km Weeks 5–8 (build): 13 → 14 → 12 (recovery) → 16 km Weeks 9–11 (peak): 17 → 19 → 20 km Week 12 (taper): 13 → 8 → race

Every 4th week drops volume by 25–30% for recovery. This 3:1 loading pattern is the single biggest predictor of injury-free training.

From week 4 onward, one weekly session becomes a brick run — a 20–40 min run immediately after a 60–120 min bike ride. This is not the long run; it's a separate session.

Brick progression:

• Week 4: 60 min bike + 20 min run
• Week 6: 75 min bike + 30 min run
• Week 8: 90 min bike + 30 min run at target race pace
• Week 10: 2 hr bike + 40 min run (dress-rehearsal)
• Week 12: skip — go into race fresh

The first 5–10 minutes off the bike feel awful. That's normal and it's what you're training. See triathlon jelly legs for the physiology.

The long run is a Zone 2 session — always. It builds mitochondrial density, capillary networks, and fat oxidation without the recovery cost of high-intensity work.

Zone 2 landmarks:

• You can hold a full sentence conversation
• Nose breathing possible for at least 30 seconds
• HR = 65–75% of max HR
• Pace = 45–75 sec/km slower than your standalone 10K PR

If your Zone 2 pace feels embarrassingly slow, you're doing it right. See heart rate vs power zones for zone-setting.

Cut long-run volume by 40% at day -10 and by 60% at day -5.

Race week schedule:

• Day -7: 90 min easy long run
• Day -5: 40 min with 4×1 min at race pace
• Day -3: 30 min easy
• Day -1: 15 min shake-out or rest
• Race day: run the plan

Overdoing the last long run is the classic mistake that leaves 2–3 minutes on the finish line. See race-week taper for the full protocol.

The #1 cause of missed 70.3 races is a stress reaction, IT band syndrome, or Achilles tendinopathy in weeks 7–10. Prevention:

1. Rotate 2–3 pairs of run shoes
2. Add 2× weekly strength (squats, single-leg RDLs, calf raises)
3. Never increase run mileage the same week you increase bike mileage
4. Recovery week every 4th week — non-negotiable
5. Sleep 8+ hours during peak weeks

If a pain persists 2+ runs at the same location, cut volume by 50% and reassess.

Split the 21.1 km into 3 sections:

km 0–7: Hold back. Target race pace + 10–15 sec/km. Legs feel dead — this is normal. km 7–14: Settle into goal race pace. Fuel every 25–30 min. km 14–21: Push. If you feel strong, gradually build pace. If you're hurting, defend pace.

Most first-timers blow up in km 12–15 by going out too fast in km 1–4. See pacing strategy by distance for detailed race-day math.