Broken 4-Hour Ironman Bike Simulation: Power Step Progression

Session Overview

A structured indoor bike session for athletes in Ironman build, using stepped power increases through a long ride to rehearse holding — and slightly building — effort as fatigue accumulates, rather than riding at one flat wattage for four hours. Best done on a turbo where you can hold precise power targets.

What You’ll Need

• A smart trainer or power meter to hold accurate wattage targets
• Your full race-day nutrition plan, fuelling on schedule throughout
• Entertainment for a long indoor session — a film, a Zwift route, or a podcast queue

Warm-Up (15 minutes)

15 minutes building gradually from easy spinning to just below your Step 1 target below.

Main Set: Power Step Progression

Four roughly equal blocks, each stepping up slightly in power, simulating the demand of holding form as fatigue builds rather than fading:

• Step 1 (60 minutes) — 60–65% of FTP, settling in and starting your fuelling schedule from the first 20 minutes
• Step 2 (60 minutes) — 65–68% of FTP, a small step up, continuing scheduled nutrition
• Step 3 (60 minutes) — 68–72% of FTP, the block where mental fatigue typically starts to bite well before physical fatigue does
• Step 4 (45 minutes) — 70–75% of FTP, finishing at or slightly above your realistic race-day target power, so the session ends at the effort you actually need to hold late in an Ironman bike leg

Cool-Down (10 minutes)

Easy spinning, dropping power gradually rather than stopping abruptly.

Coaching Notes

• Adjust the four power steps to your own realistic race-day target rather than these exact percentages — the point is a gentle step-up, not hitting specific numbers that don’t match your fitness
• This is as much a mental rehearsal as a physical one: Step 3 is deliberately the hardest part psychologically, mirroring hour three of an Ironman bike leg when the finish still feels distant
• Scale down to 3 x 45-minute steps (roughly 2.5 hours total) if 4+ hours indoors isn’t realistic yet, and build toward the full version over your build phase
• Treat this as a full rehearsal of race nutrition — see our gut-training guide if anything feels off during the later steps