Winter is where the triathlon bike leg is actually built. Not March, not the week of the race. It's the 12 weeks between late November and late February, when the engine either gets bigger or it doesn't.
The problem: for most age-groupers in Ireland, the UK, and northern Europe, outdoor riding in those months is a mix of cold, dark, wet, and compromised. Two hours outside in 4°C and rain produces less usable training stimulus than 75 minutes on a smart trainer with a proper session.
This is the full indoor plan. Twelve weeks, three phases, structured around what actually moves a triathlete's bike split, not what looks good on Strava.
Why winter is the season that decides your bike split
The bike leg is the longest portion of any triathlon. In a 70.3 that's two and a half hours of work for most age-groupers. In an Ironman it's five to seven. Whatever FTP and aerobic durability you bring to race day was mostly built 16–24 weeks earlier.
Dan Lorang, who coached Jan Frodeno and Anne Haug through multiple Ironman world titles, has been explicit on the Roadman Cycling Podcast about this: the winter block is where the aerobic system is stressed hard enough to adapt, because spring and summer are too cluttered with racing, tapering, and recovery to build capacity. You maintain in-season. You build off-season.
Prof. Stephen Seiler's polarised training research points the same direction. Roughly 80% of weekly training time should sit below the first lactate threshold, with 20% above the second. Indoor winter training is actually where this ratio is easiest to enforce, because the trainer holds you exactly where you programme it to.
The mistake most age-groupers make is treating winter as maintenance: two or three short, unstructured rides, no progression, no measurement. Then they arrive in April with the same FTP they had in October, and the season is already decided.
A structured 12-week indoor block, built around triathlon bike coaching principles, typically adds 15–25 watts to FTP for an age-grouper starting from a maintained base. That's 2–4 minutes off a 70.3 bike split at the same perceived effort, with no cost to the run.
The indoor-cycling bike-fit reset
Before week one of the plan, sort the fit. Indoor riding exposes fit problems outdoor riding hides: the road doesn't move, you don't coast, and you don't shift position to descend or corner. Eight hours a week in a compromised fit produces injuries, not fitness.
Three things to check. First, saddle height and setback on the TT bike should match your road bike within 2–3mm of effective seat position. Most age-groupers run their TT saddle too low, which kills power in the aero bars. Second, aero bar pad height. If you can't hold the position for 20 minutes on the turbo without neck pain, the pads are too low or too far forward.
Third, cleat position. Indoor riding increases total pedal strokes per session compared to outdoor riding of the same duration, because there's no coasting. Minor cleat misalignment that you'd never notice outside will show up as knee pain by week four.
Get your FTP zones set correctly before starting the plan. Retest at the end of week four and again at week eight. A 20-minute test, or a ramp test if you have no baseline, will work, as long as you use the same protocol each time.
Set the trainer up in a room under 18°C with a fan rated at 3,000+ CFM. Cardiac drift on the turbo is almost entirely a cooling problem. Dan Bigham, the former UCI Hour Record holder, has been clear that heat management is the single biggest limiter of sustained indoor power, more than hydration, more than fuelling.
Weeks 1–4: Aerobic base (indoor edition)
The first block is boring and essential. Four weeks of Zone 2 — 65–75% of FTP, conversational effort, heart rate stable. The goal is mitochondrial density and fat oxidation, not peak power.
Structure: three to four rides per week, 5–7 hours total. One long ride of 90–120 minutes at steady Zone 2. Two rides of 60–75 minutes, one with 3–4 short tempo blocks of 5 minutes at 85% FTP to keep the legs awake. One optional 45-minute easy spin.
Cadence variation matters here. Spend 20% of each session at 70–75 rpm (big-gear, low-cadence strength work at Zone 2 power) and 10% at 95–100 rpm. This replaces the cadence variety that outdoor terrain provides naturally.
Start building TT position tolerance. In week one, hold the aero bars for 10 minutes of each ride. By week four, you should be able to sit in position for 45 minutes of the long ride at steady Zone 2 power without dropping watts or lifting your head.
Don't test FTP in week one. Test at the end of week four. Prof. Seiler's lab has shown repeatedly that the adaptations from Zone 2 work compound over three to four weeks. Testing earlier measures noise, not progress.
Fuel the long rides. 60–80g carbs per hour on any ride over 90 minutes, even in winter. Tim Spector's work on metabolic flexibility doesn't mean training fasted. It means building the aerobic engine that lets you use fat efficiently when you need to. That engine requires volume, and volume requires fuel.
Weeks 5–8: Threshold and VO2max
Now the hard work. Four weeks of targeted intensity, built on the aerobic base from block one.
Two quality sessions per week, separated by at least 48 hours. Keep the long Zone 2 ride. Drop total volume slightly: 5–7 hours is enough when two of those hours are hard.
Threshold session (once per week): 2x20 minutes at 90–95% FTP with 8 minutes recovery, progressing to 3x15 at 92–96% by week seven. Hold aero bars for the full interval from week six onward. This is the single most important session for 70.3 and Ironman bike performance — sustained sub-threshold work in race position.
VO2max session (once per week): 5x3 minutes at 110–118% FTP with 3 minutes recovery, progressing to 6x3 by week seven. Don't worry about aero position here. Sit up, breathe, produce power. VO2max lifts the ceiling that threshold work pushes against.
John Wakefield of Red Bull-Bora-Hansgrohe has pointed out that World Tour riders get most of their gains from sustained threshold work, not from short, sharp efforts. The same applies to triathletes, more so, because the race demand is sustained, not punchy.
Retest FTP at the end of week eight. Expect 8–15 watts of improvement if block one was executed properly. If the number hasn't moved, the problem is almost always insufficient Zone 2 volume in weeks 1–4, not insufficient intensity here.
Run training stays protected. This is the rule: bike intensity never compromises the key run session. If the Tuesday threshold ride kills the Wednesday run, the bike load is too high.
Weeks 9–12: Race-specific aero endurance
The final block translates fitness into race performance. The difference between a high FTP and a fast bike split is the ability to hold a high percentage of that FTP, in aero position, for the full race duration.
Primary session: race-pace sustained efforts. For 70.3 athletes, 2x40 minutes at 78–82% FTP in full aero position with 10 minutes recovery, building to 1x90 minutes by week eleven. For full-distance, 2x45 at 70–75% FTP building to 1x2 hours at 72–76% by week eleven.
Keep one threshold or VO2max session per week to maintain the ceiling: 2x15 at 92% FTP is enough. Keep the long Zone 2 ride but shift 60–90 minutes of it into aero bars.
Brick runs become critical. Finish the weekend long ride with a 20–30 minute run at race pace. Indoor riding followed immediately by a run trains the neuromuscular transition better than outdoor riding, because you can step off the trainer and start running in 90 seconds.
Nutrition rehearsal matters here. Practise race-day fuelling on every long ride — the exact gels, bars, and bottles you'll use on race day, at the exact rate. Joe Friel has written that most Ironman bike legs fall apart from fuelling errors, not fitness errors. The last four weeks is when you fix that.
By the end of week twelve, you should be able to hold race-pace power in aero position for the full race bike duration, finishing with enough in reserve to run well. That's the product of the block.
How to avoid indoor overtraining
Indoor training has a higher injury-to-fitness ratio than outdoor training if you're not careful. Three rules.
First, cap hard sessions at two per week for 12 straight weeks. A third hard session every week accelerates FTP gains for about four weeks, then stalls completely and compromises the run. Two is the sustainable dose.
Second, track morning resting heart rate and HRV. A 10% drop in HRV from baseline, or a 7+ bpm rise in resting heart rate, for three consecutive mornings means swap the next quality session for Zone 2 or rest. Don't negotiate with the data.
Third, manage the environment. Room under 18°C, large fan, 500–750ml fluid per hour with electrolytes. Indoor cardiac drift is heat, not fitness. If your heart rate rises 15 bpm across a 90-minute Zone 2 ride at stable power, the room is too warm.
The concrete next step: set a retest date for the end of week four, programme the three sessions for week one tonight, and move the trainer somewhere cold enough to ride hard in.
