How to run 21.1 km on legs already worked by 90 km of cycling without falling apart: pacing from step one, heat management and the transition.
"Why do my legs always fall apart in the last kilometers of the half marathon?" Honest answer: usually not because you lack fitness, but because the bike leg before it was too hard or your fueling wasn't right. A 21.1 km run after 90 km on the bike is a completely different task than a standalone half marathon — your legs don't start fresh, they start already fatigued from more than two hours of riding.
The most common reason for a bad run: overpacing in the first 30–45 bike minutes or under-fueling on carbs — both usually happened chapters earlier but only show their effect now. The recommendation from experienced coaches: start the run deliberately easy, closer to your pure marathon pace or even slightly slower, not at your half-marathon PR pace.
Heat makes the problem worse. In cycling time trials, output drops by an average of 15% at temperatures around 30°C and above. Running shows it over time: for every degree above roughly 17°C, finish time worsens by an estimated 0.2 to 1.1%. That sounds small, but over 21 km it quickly adds up to several minutes.
Right after dismounting, legs often feel heavy or numb — that's normal and usually passes after the first 500–800 meters. Force the pace here and you make the problem worse instead of solving it.
Coaches recommend starting the run off the bike closer to your marathon pace or slightly below it, rather than pulling your full half-marathon pace right away. That protects against the classic fade in the second half.
Don't simply cut back your carb intake just because you're running now — your body still needs energy. Switch to more easily digestible gels and small sips rather than solid food.
Use aid stations for water over your head and neck, not just for drinking. External cooling lowers core body temperature without you having to change pace — a simple, underrated lever against heat-related fade.
If you run the first 5 km stable and easy, you may carefully pick up the pace. If it already feels hard there, holding is more important than pushing — a negative split (faster second half) rarely works if the first half was already too aggressive.
A run fade is unpleasant but usually harmless. Not so with real heat-exhaustion signs: dizziness, confusion, ceasing to sweat despite heat, or severe nausea are alarm signals — slow down immediately, cool down, and contact medical staff if in doubt.
Don't just drink indiscriminately more in heat without also replacing sodium — that raises hyponatremia risk further (see the fueling guide). And listen to your body: a controlled race DNF is always better than a circulatory collapse on course.
Deliberately plan a "reset phase" for the first running kilometers: light, relaxed jogging, a quick shoulder and arm shake-out, deep breathing. That 500 meters of "lost" time is almost always won back several times over in the rest of the race, because you don't run straight into a fade.
A trick experienced finishers use: consciously count how often you cool and drink at aid stations while running, rather than leaving it to chance — a fixed routine beats spontaneous decisions once your head gets tired under exertion.
That's a well-known effect of the muscular shift from cycling to running load and normally not a warning sign. It usually settles after the first 500–800 meters of easy jogging — exactly why experienced athletes deliberately start the first kilometers calm.
Closer to your pure marathon pace or slightly below it, not at full half-marathon pace. That protects against the classic fade in the second half of the race, which usually stems from too aggressive a start.
Studies show a worsening of roughly 0.2 to 1.1% per degree above about 17°C. Over 21.1 km that can quickly add up to several minutes — active cooling at aid stations noticeably counters this.
If your legs feel excessively heavy already in the first kilometers instead of normalizing after 500–800 m, or you're running noticeably slower than in training at the same effort, the bike leg was probably too hard.
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