Two runners line up with the same VO2 max, the same long-run history, and the same goal time. Three hours and forty-five minutes later, one of them crosses in 3:45 with a smile and the other limps in at 4:02 with the look of a person who has rethought their life choices somewhere around 32K. The difference is almost never fitness. It's pacing.
The marathon punishes mistakes that the half marathon forgives. A 10-second-per-kilometre overshoot in the first 10K of a half-marathon costs you a slightly hard finish. The same overshoot in a marathon empties your glycogen stores 20 minutes before the line and turns the last 10K into a walk-jog-grimace cycle. This guide is about avoiding that — and, if you're ambitious, running the rarest and most satisfying race shape there is: a marathon negative split.
Why the marathon punishes bad pacing
Your body stores roughly 1,800–2,000 calories of glycogen between your liver and your working muscles. At marathon effort, you burn somewhere around 70–100 calories per kilometre depending on your size and economy. Do the maths and you'll see why pacing matters: a well-fuelled, well-paced runner finishes a marathon with the tank scraping the bottom but not empty. A runner who goes out too hard burns through their carbohydrate stores faster, runs out around 30–35K, and is forced to switch to fat as the dominant fuel.
Fat is a slower fuel. Your pace drops sharply, your perceived effort spikes, and your form falls apart. That's "the wall." It isn't a mystical event — it's a metabolic one, and it is almost entirely driven by how you spent your energy in the first half of the race.
The other half of the puzzle is muscular. Every kilometre you run faster than goal pace early on costs you disproportionately more eccentric muscle damage in your quads. By 30K, that damage shows up as a leg that simply won't extend the way it used to. Pacing well isn't just about saving fuel — it's about saving the legs that need to carry you home.
The three pacing strategies
Every marathon you run will fall into one of three shapes:
- Even splits: first half and second half within roughly 30 seconds of each other. The classic textbook race.
- Negative splits: second half faster than the first. The hallmark of a well-judged effort and almost every recent men's marathon world record.
- Positive splits: first half faster than the second. The default of nearly every amateur marathon, almost always by accident.
Most marathon world records have come on either even or slightly negative splits. Eliud Kipchoge's 2:01:09 Berlin world record in 2022 was a negative split — 60:48 first half, 60:21 second. And on April 26, 2026, Sabastian Sawe ran 1:59:30 in London with a 60:29 first half and a 59:01 second half — an 88-second negative split at world-record pace, including a closing 2.195K in 5:51.
The amateur picture is the mirror image. Pull the data from any major marathon and you'll find that something in the order of 70–80% of finishers run positive splits, often by 5–15% — meaning a runner targeting 3:45 (5:20/km) might run the first half at 5:10/km and the second at 5:35/km, finishing several minutes off goal.
Even splits: the safe baseline
An even-split marathon is what every pacing chart, race-day wristband, and goal-pace calculator implicitly assumes. You pick a target pace, you run that pace from 1K to 42K, and the watch ticks over at the right time. It is the simplest plan, and for first-time marathoners it is genuinely the right one.
The catch is that "even effort" and "even pace" are not the same thing. The first 5K of a marathon at goal pace will feel laughably easy. The last 5K of the same race, at the same pace, will feel like the hardest thing you've ever done. Even-pace racing requires you to override the early easy feeling and resist speeding up — which is harder than it sounds with adrenaline running and 30,000 people around you.
Negative splits: how the best races are run
A negative split means you cover the second half faster than the first. Done well, the difference is small — 30 to 90 seconds. Done very well, it means you finish the race accelerating, passing people, and crossing the line with the rare feeling that you got the most out of the day.
It works for two reasons. The first is physiological: by starting 5–10 seconds per kilometre slower than goal pace, you preserve glycogen, keep your heart rate lower, and arrive at 30K with the muscular and metabolic reserves to actually run hard. The second is psychological. In a positive-split marathon, every kilometre after halfway is harder than the last and you're slowing down — which is demoralising. In a negative-split marathon, every kilometre after halfway you're passing people who went out too fast, which is the most powerful motivational drug in the sport.
How to actually execute a negative split
Telling someone to "just run the second half faster" is useless advice. Here's the breakdown that actually works for a sub-elite runner targeting a 3:30 to 5:00 marathon:
- 0–10K: Run 5–10 seconds per kilometre slower than goal pace. This is the hardest part — it will feel painfully easy and everyone around you will be ahead. Let them go. You will see most of them again.
- 10–30K: Settle into goal pace. Not faster. The 20K block in the middle of the race is where you bank the consistency that the final 12K will demand.
- 30–42K: Now you can spend. Aim to run 5–10 seconds per kilometre faster than goal pace from 32K onwards. If you've paced the first 30K correctly, this will feel hard but possible. If you haven't, it will feel impossible — and that's useful feedback for next time.
Two extra things worth knowing. First, "5–10 seconds slower" early doesn't mean 30 seconds slower — that's a different race. Second, if 32K arrives and you can't lift the pace, hold goal pace and finish strong. A near-even split run with discipline still beats a positive split run with bravado.
Common pacing mistakes
Most amateur positive splits are caused by the same handful of errors, in roughly the same order of severity:
- Going out too fast. Adrenaline, downhill starts, and a fresh body conspire to make goal pace feel slow in the first 5K. The fix is to consciously hold back — if your first kilometre isn't slower than goal pace, you've already made the most common mistake in the sport.
- GPS drift in cities. Tall buildings, tunnels, and bridges throw off watch GPS by 1–3% in major city marathons. Runners who chase a watch pace in central London, New York, or Berlin often run 10–15 seconds per kilometre too fast without knowing it. Use the official kilometre markers as the truth and treat the watch as a rough guide.
- Chasing the pack. Marathon pace groups are useful but imperfect. If the 3:30 group goes through 5K in 24:30 instead of 24:53, they're already 23 seconds ahead of even pace. Run your race, not theirs.
- Aid station chaos. Sprinting to grab a bottle, then sprinting again to get back into rhythm, costs more energy than holding a steady line and missing the occasional drink. Slow into the station, walk a few steps if you have to, and resume pace smoothly.
- Treating downhills as free speed. Aggressive downhill running early in the race trashes your quads. By 30K you'll wish you'd jogged them.
A worked example: 3:45 marathon, three ways
A 3:45 marathon is 5:20 per kilometre on the nose. Here's what each pacing strategy looks like in practice, broken into 5K splits and a final 2.195K. Same finish time on the even and negative versions; the positive version is what happens when the early pace runs ahead of the plan.
| Segment | Even splits (3:45:00) | Negative splits (3:45:00) | Positive splits (4:01:30) |
|---|---|---|---|
| 0–5K | 26:40 | 27:15 | 25:30 |
| 5–10K | 26:40 | 27:00 | 25:45 |
| 10–15K | 26:40 | 26:45 | 26:15 |
| 15–20K | 26:40 | 26:40 | 26:45 |
| 20–25K | 26:40 | 26:40 | 27:30 |
| 25–30K | 26:40 | 26:35 | 28:30 |
| 30–35K | 26:40 | 26:25 | 30:00 |
| 35–40K | 26:40 | 26:15 | 31:00 |
| 40–42.195K | 11:42 | 11:25 | 14:15 |
| Finish | 3:45:00 | 3:45:00 | 4:01:30 |
The even-split runner ran 5:20/km the whole way. The negative-split runner started at 5:27/km, settled to goal pace by 15K, and finished the last 12K at 5:14/km. The positive-split runner ran the first 10K at 5:07/km — 13 seconds per kilometre too fast — and paid a compounding tax for it from 25K onwards. All three were equally fit on the morning of the race. Only the pacing was different.
Plan your race before you run it
The marathon is not a race you wing. Decide your strategy a week out, write your 5K target splits on a wristband or piece of tape on your watch strap, and rehearse the first 10K in your head: easy, easy, easy. Trust that the easy start is the down payment on the strong finish.
You can model your own marathon splits — even, negative, or positive — with the calculator at runpace.co. Adjust the per-kilometre pace, use the negative-split generator to sketch a finish-strong plan, and print or screenshot the splits to take to the start line. The runners who finish well are almost always the ones who decided how they'd run before the gun went off.