The short answer
A useful IRONMAN 70.3 brick teaches you to begin running after cycling without turning the session into a test. Start with 10–15 minutes of easy running after a normal ride, then progress duration or specificity one variable at a time.
Research on the cycle-to-run transition reports physiological, biomechanical, and sensory changes after cycling, and a small field study found reduced running distance and shorter stride length after a bike segment compared with an isolated run.1 The exact response differs by athlete. That is why a brick is a rehearsal, not a universal pace prescription.
IRONMAN’s own preparation material recommends practising bike-to-run transitions.3
Four types of brick—and their jobs
| Brick | Example | Job |
|---|---|---|
| Transition brick | Normal ride + 10 min easy run | Learn shoes, posture, and the first kilometre |
| Aerobic brick | Steady ride + 20–40 min easy run | Build repeatable durability |
| Race-specific brick | Race-effort bike segments + controlled run | Test pacing and fueling decisions |
| Logistics brick | Short bike + short run with full transition setup | Rehearse equipment and sequence |
Do not combine all four jobs every week.
An eight-step progression
Use one step for one or more weeks. Progress only when recovery and the following sessions remain stable.
- 10 minutes easy after an easy ride. Ignore pace; settle breathing and cadence.
- 15 minutes easy after a longer ride. Practise changing shoes and starting calmly.
- 20–25 minutes easy. Keep the first five minutes deliberately restrained.
- Add three short controlled pickups. These are not sprints.
- Practise race fueling on the bike. Change nothing else.
- Add race-effort bike blocks. Keep the run easy.
- Add a short race-effort section to the run. Only after the bike pacing is repeatable.
- Complete one bounded dress rehearsal. Finish knowing what to change, not needing a recovery week.
The first-kilometre rule
The first kilometre should feel almost too controlled. Use it to check:
- Is breathing settling or escalating?
- Can you shorten stride without shuffling?
- Did bike fueling leave your stomach manageable?
- Are shoes, laces, socks, and kit working?
- Is any pain changing your gait?
If the first kilometre is faster than the pace you can sustain, the brick practised the wrong decision.
A brick review card
After the session, write five lines:
- final 30-minute bike effort and conditions;
- fuel and fluid taken;
- transition time and any equipment friction;
- first-kilometre effort, not just pace;
- how the next day’s easy training felt.
This converts a hard-looking session into reusable information.
Where bricks belong
Most weeks do not need a long or hard brick. A short transition run can follow a ride without adding much complexity. Race-specific bricks deserve more recovery and should not sit beside the hardest long run or heavy lower-body strength work by accident.
Use the IRONMAN 70.3 training hub for the complete cycle and the training start-date calculator to see the runway to your event.
Safety and uncertainty
Practise on safe routes and follow traffic rules. Heat, terrain, wind, bike fit, fueling tolerance, training history, and health all change the cost of a brick. Stop for chest pain, fainting, severe breathing difficulty, confusion, or acute injury. Persistent pain or gastrointestinal problems deserve qualified assessment.
Frequently asked questions
How long should my first brick run be?
Ten easy minutes is enough to learn from. Duration is not the measure of quality.
Should the run be at race pace?
Usually not at first. Establish a controlled transition, then add short race-specific work later.
Do I need a brick every week?
Not necessarily. Frequency depends on phase, experience, recovery, and what the session is meant to teach.
Can I do a run-to-bike brick?
It can serve a training purpose, but it does not rehearse the event’s bike-to-run transition.
This guide is educational, not medical advice. Stop and seek appropriate care for chest pain, fainting, severe breathlessness, or pain that changes your movement.
26weeks.ai keeps swim, bike, run, recovery, and disrupted weeks connected to the same finish-line plan. Start training with 26weeks.ai.
References
- Millet GP, Vleck VE. Physiological and biomechanical adaptations to the cycle-to-run transition. British Journal of Sports Medicine. 2000. (PubMed)
- Bini RR, et al. Effects of cycling on subsequent running performance. Sports. 2019. (PubMed)
- IRONMAN. IRONMAN 70.3 pre-race preparation checklist. (Official checklist)