Why runners blow up at km 15–18
The half marathon wall — that sudden collapse in pace around km 15–18 — is almost always the result of going out too fast in the first 5 km. Race day adrenaline, crowd energy, and fresh legs conspire to make goal pace feel easy for the first few kilometres. It isn't. You're burning through glycogen faster than your aerobic system can replenish it.
By km 10 you feel fine. By km 15, the deficit catches up. Your legs feel heavy. Your pace drops. This is not a fitness problem — it's a pacing problem. Runners who suffer this consistently in races are almost always capable of the time they're targeting; they just spend it in the first half.
The fix is mechanical: run the first 5 km 10–15 seconds per kilometre slower than your goal pace, regardless of how easy it feels. This is the single most effective race day change most amateur runners can make.
How to pace the first 5 km
Start at the back of your target pace corral, not the front. This automatically adds 15–30 seconds to your first kilometre split as you navigate around slower runners — which is exactly what you want.
Check your watch at km 1. If you're on goal pace or faster, deliberately slow down. Your target for km 1–5 is goal pace plus 10–15 seconds per kilometre. If your goal is 5:41 min/km (sub-2 hours), your first 5 km should be at 5:50–5:55 min/km.
This will feel too slow. Other runners will pass you. Ignore them. The runners who pass you in km 2 are the runners you will be passing in km 17.
- →Km 1–5: goal pace + 10–15 seconds/km (controlled, conversational effort)
- →Km 6–12: goal pace, settling into rhythm
- →Km 13–17: goal pace or slightly faster if feeling strong
- →Km 18–21: give everything that's left
Fuelling strategy: what to eat and drink
For most runners completing a half marathon in 1:45–2:30, one to two gels or equivalent during the race is sufficient. The half marathon is largely a glycogen-sustained effort — unlike the marathon, you don't need to replenish aggressively. But you do need to take on something.
Take your first gel at km 8–10, before you feel like you need it. Gels take 10–15 minutes to enter your bloodstream, so waiting until you feel empty at km 12 means they won't help until km 14 at the earliest. By then, the damage is done.
Water stations: drink a small amount at every station from km 5 onward. Don't try to drink large amounts at once — slow to a walk for 5 seconds if needed. Dehydration in a half marathon is rare in cool weather; overdrinking is more common. Sip, don't chug.
Race week and race morning preparation
The week before: run less, sleep more. Your fitness is already set. Nothing you do in the final 7 days will meaningfully improve your performance, but insufficient sleep will meaningfully hurt it.
Two nights before the race matters more than the night before. Race nerves often disrupt the night before — this is normal and not damaging if you slept well two nights prior.
Race morning: eat a familiar breakfast 2–3 hours before the start. Nothing new. Carbohydrate-based, easy to digest. The classic choice is porridge, toast with banana, or a bagel. Avoid high-fat, high-fibre foods that slow gastric emptying.
- →7 days out: reduce volume to 50–60% of normal, maintain one short quality session
- →2 nights before: prioritise 8 hours sleep
- →Race morning: familiar breakfast 2–3 hours before start
- →Race morning: light warm-up walk 15 minutes before start
- →At the start: seed yourself 10 seconds behind your target corral
How to handle hills, wind, and heat on race day
Adjust effort, not pace, on hills. Running uphill at your target pace requires significantly more energy than running flat at the same pace. If you maintain effort on the climb, you arrive at the top with capacity to run the descent and the following flat. If you maintain pace, you'll be breathing too hard to recover.
Run into wind at slightly reduced pace on the way out, bank the energy for the return. Don't try to fight a headwind — the aerobic cost of maintaining pace into a strong wind is disproportionate to the time saved.
In heat above 18–20°C, adjust your goal by 30–60 seconds per kilometre depending on conditions. Heat increases your heart rate for a given pace — running at your normal goal pace in hot weather means you're working significantly harder than in training. Adjust the goal, not the effort.