Most runners who have tried a marathon have experienced the same thing: they felt great at 20km, less great at 30km, and by 35km they were wondering why they ever signed up.
The most common reason is pacing. Specifically, going out too fast.
If you have got a marathon coming up and you are still figuring out how to approach the race, here is the one idea that most experienced runners eventually come around to: run the second half faster than the first.
That is a negative split. And it works.
What a negative split actually means
A negative split means your total time for the second half of the race is faster than the first half.
It does not mean a dramatic difference. An extra 1 to 3 minutes on the back half is enough to count. What it means practically is that you hold back in the early kilometres, run deliberately below your goal pace, and build into the race rather than chasing time from the gun.
Most runners do the opposite. They go out at goal pace or faster, feel comfortable, and then gradually slow as fatigue builds. By the time the 30km mark arrives, the legs have turned, the pace has slipped, and the last 12km becomes a test of how much they can hold on.
Negative splitting reverses that. Instead of hanging on, you are working into it. You pass people. You finish strong.
Why it works
The biology behind negative splitting comes down to glycogen and lactate.
When you run at a pace above your aerobic threshold, your body produces lactate faster than it can clear it. Glycogen (stored muscle fuel) gets depleted faster too. The earlier you tip into that zone, the sooner things start to fall apart.
By starting slightly slower, you preserve glycogen for the kilometres when you need it most, keep lactate from building too quickly, and stay in a metabolic zone where you can actually sustain effort. The result is that you have more left in the second half, not less.
Most world marathon records have been set with a slight negative split. That is not a coincidence.
How to actually run one
A simple way to think about pacing a negative split marathon is to divide the race into three sections.
The first 15km: run around 10 to 15 seconds per kilometre slower than your goal pace. Expect to feel uncomfortably restrained. That is the point. The body is fresh, adrenaline is high, and everything will feel too easy. Resist it.
Kilometres 15 to 30: settle into your goal pace. You should start to feel the effort here, but it should feel controlled and sustainable. Focus on running cadence and staying relaxed through the shoulders and arms.
The final 12km: this is where you decide what kind of race you want to run. If you have been disciplined in the first two phases, you will have something left here. Not a sprint, but enough to hold pace and, if things go well, gradually increase effort through to the finish.
No complicated formula. Just three zones and a commitment to starting slow.
Keep an eye on form, not just pace
One thing that gets overlooked in marathon pacing is that pace on its own does not tell the full story. Running form starts to deteriorate in the final third of a marathon for most runners, and that is often where time is lost.
Keeping a mental checklist, posture upright, arms relaxed, foot landing under the hips, helps you stay efficient when fatigue sets in. It is also worth thinking about form during your speed work sessions in training, because the habits you build in training carry through to race day.
Fuel early, not just when you are hungry
A negative split strategy only works if your fuelling matches it. The biggest mistake runners make is waiting until they feel depleted before taking on carbohydrates. By that point, you are already behind.
Take your first gel or carbohydrate source around the 40 to 45 minute mark, before you feel like you need it. Then continue to fuel regularly throughout the race, roughly every 45 minutes. The goal is to keep blood glucose steady, not to recover from a crash.
And make sure whatever gels or chews you use on race day are ones you have tested in training. Race day is not the time for new products.
What to think about for race week
If you are in the final week before your marathon, the main job is to protect the fitness you have built, not add to it. Reduce volume, keep a couple of short sharp efforts to stay sharp, sleep well, and eat consistently. Do not try anything new with food, shoes, or gear.
Plan your pacing on paper before race day. Know your target kilometre splits for each phase (conservative, goal, and final push). Write them on your arm if you need to. It sounds simple, but having the numbers in front of you removes one decision when your brain is already working hard.
Gear that helps
You do not need much for a marathon, but getting the basics right removes friction on race day.
GPS watch or pace app: Real-time pace feedback is the single most useful tool for executing a negative split. It stops you from running on feel in the early kilometres when everything feels deceptively easy.
Well-tested shoes: Race day is not the day for new shoes. Whatever shoes you have been training in, or a version you have put at least a few long runs on, are your best option. Carbon plate shoes can help, but only if you have trained in them.
Running sunglasses: Early marathon starts mean you are often running in multiple light conditions, from low pre-dawn light into full daylight over the course of 3 to 5 hours. A photochromic lens adjusts automatically as light changes so you are not squinting into the sun at 8am or fumbling to swap lenses mid-race.
The Adaptor lens is photochromic, going near-clear in low light and darkening progressively as the sun rises over the course of your race. It is the most practical option for variable conditions and changing light. For runners who want the full setup, the Infinity lens combines photochromic with polarised and anti-fog, so it handles humidity, glare, and brightness all at once. Both are built specifically for running, so they sit securely across the full distance. The lens guide can help you pick the right one for your race conditions.
Lubrication: Chafing does not announce itself until kilometre 25. Apply Body Glide or Vaseline anywhere fabric contacts skin before you start. Small thing, matters a lot after three hours of movement.
What to expect
The first time you deliberately run a negative split, it will feel wrong. The restraint in the early kilometres feels like giving up time. Other runners will pass you. Your watch will show a pace slower than you know you can hold.
But by 28km, that restraint pays off. The effort feels manageable. The pace holds. And on a good day, you finish the last 5km faster than you ran the first.
That is the feeling most experienced marathoners are chasing. It starts with going out slow.
If you are preparing for an upcoming race, the race day guide covers the full morning timeline from warm-up to crossing the finish. And if you are running the Sydney Marathon this month, the Sydney Marathon course breakdown maps every hill so you can plan exactly where to push and where to hold back.
Go out slow. Finish strong.
Tim Golubev
Founder, Re. (Re Your Run)
Tim built Re. after years of running in sunglasses that bounced, fogged, and ended up on his forehead. After discovering the UV damage that builds up without eye protection (even on cloudy days) and hearing the same frustrations from hundreds of other runners, he decided it was a problem worth fixing properly. With a background in Product across multiple industries, he approached it like any product problem: figure out what's broken, then build something that actually fixes it. He runs daily, co-founded Rose Bay Run Club, and Re. is his attempt to make one less thing that gets in the way of a good run.
