Most marathons are lost in the first 10km, not the last. Here is how to pace one from start to finish: your splits, your effort, and what to do when it hurts.
By Alex · 8 min read
The marathon is the only race where the most dangerous moment is the first one. Start 10 seconds per kilometer too fast and you will pay for it from kilometer 30 onward. Get it right and you will pass 30 runners in the last 5 kilometers.
This guide is about getting it right.
Think about your last race. Did you feel great at halfway? And how did you feel at 35 km? The difference between those two feelings usually depends on how fast you ran the first 10 km.
The crowd is loud at the start. Your legs feel fresh. Your training is finished. Everything tells you to go. And if you go, you will pay for it.
The "wall" at 30–35 km happens when you run out of stored carbohydrate (glycogen). Early pace mistakes make that happen sooner. Running 15 seconds per kilometer too fast in the first half burns your glycogen more quickly and leaves you empty at exactly the moment you need the most.
In the first 10 km of a marathon, your correct target pace feels easy. Strangely, almost worryingly easy. Runners who have never felt this ignore the GPS watch and run by feel, and they end up running too fast.
That is why you need a plan before race day.
There are two standard ways to pace a marathon:
Even split: the same pace for every kilometer of the 42.195 km. This takes a lot of discipline in the first half, and it usually gives the most reliable results for recreational runners.
Negative split: run the second half faster than the first. The best marathon times are usually run with a small negative split (1–2 minutes faster in the second half). For recreational runners, aiming for an even pace often produces a natural negative split anyway, because most people start carefully.
For most runners, aiming for even splits and finishing with a small negative split is the best strategy.
Start with your goal finish time. Use the pace calculator to find your exact split per kilometer or mile.
For a 3:30:00 marathon:
Write these splits on your wrist, set pace alerts on your watch, and stay with them for the first 25 km, however you feel.
If you are chasing a big personal best, there is a simple rule: if you feel comfortable, you are running correctly. If you feel great at 20 km, something is wrong. If you feel in control — working, but under control — you are on target.
For most runners the discomfort arrives at about 28–32 km. If it comes earlier, you started too fast. If it never comes, you had more to give.
Your body is warming up. Your glycogen stores are full. Your legs feel bouncy. This is when the race feels easiest, and it is when most of the damage is done.
Run 5–10 seconds per kilometer slower than target pace. If your goal is 5:00/km, run 5:05–5:10. You get that time back in the second half, when other runners slow down.
This is your cruising phase. Find your rhythm. Hit your target pace again and again. Do not react to runners passing you or to the crowd. Just follow the plan.
This is where the race really starts. Your glycogen is running low. Your legs are getting heavy. Every kilometer feels like an argument with yourself.
Now think about effort rather than pace. Keep the same effort level you have had since kilometer 10. Your pace may drop a little, and that is fine. What matters is keeping the effort honest.
This is also when fueling mistakes catch up with you. If you have not taken in enough carbohydrate all race, you will find out at about kilometer 30.
Pace does not matter now. Only moving forward matters. Shorten your stride if your form starts to fall apart. Run the tangents. Count to 100. Focus on the next runner ahead. Do whatever gets you to the finish line.
If you have paced the race well, your last 7 km will be among the fastest you run all day.
Hot, humid weather means you must slow down. A simple guide:
Ignoring the weather is a common reason runners fall apart late in a race.
Pacing and fueling go together. Even a perfect pacing plan can fall apart if you do not take in enough carbohydrate. For most marathons:
Use the Fuel Planner to work out how many gels to carry, when to take them, and which aid stations to use.
The night before:
Race morning:
When it gets hard (it will):
The best tool I know for race preparation is the pace calculator. Enter your goal time, get your target pace per kilometer, and write it on your wrist.
If you have a recent race time (5K, 10K or half marathon), you can also use the VDOT race prediction to check whether your marathon goal is realistic. A goal time that is too ambitious is the most common reason marathons go wrong.
Calculate your marathon splits →
Every page on this site has a Markdown mirror at the same path with a .md suffix (for example /calculator.md), and honors an Accept: text/markdown request header on the HTML URL. The calculations here are also available as callable tools through a free public MCP server — no account or API key.