How to Pace a Marathon: Even Splits, Negative Splits and the Riegel Reality Check

Almost every first marathon goes the same way: the first 15 km feel easy, the crowd is loud, the legs are fresh, and you bank a little time “for later.” Then somewhere past 30 km the wheels come off, the pace collapses, and the last 10 km become a grim shuffle. The cruel arithmetic is that the seconds you “saved” early cost you minutes late. Good marathon pacing is mostly the discipline to run slower than you want to for the first half — and the way to make that feel safe is to have a number you trust.

Start from a race you’ve actually run

Your marathon goal pace shouldn’t come from hope; it should come from a recent shorter race. The standard tool for that job is the Riegel formula, which scales a known time at one distance into a predicted time at another — the arithmetic, and why it works, are pulled apart in the running pace and race-time prediction guide. Here you only need the result: run a 50-minute 10K and Riegel predicts roughly a 3:52 marathon — if your endurance is fully built. That “if” is the whole story, which we’ll come back to.

The Running Pace calculator does this prediction for you: enter a recent race time and distance and it returns projected times for the 5K, 10K, half and full marathon, plus the exact min/km and min/mi pace each one implies. That predicted marathon pace is your ceiling, not your target — more on that next.

Why the predictor lies to beginners (and how to correct it)

Riegel assumes your training supports the longer distance. For a seasoned runner logging 60–80 km a week, the marathon prediction from a half is usually within a couple of minutes. For someone whose longest run was 30 km and whose weekly mileage is modest, the raw prediction is optimistic — endurance, not leg speed, is the limiter, and the formula can’t see your training log.

A practical correction: take the predicted marathon time and add 3–6% if your mileage is low or this is your first. On a 3:52 prediction that’s roughly 7–14 minutes — the difference between finishing strong and walking the final 5 km. It feels like leaving time on the table. It isn’t; it’s insurance against the second-half collapse that costs far more.

Even splits vs negative splits

There are three ways to distribute your effort across 42.195 km:

• Positive split — first half faster than the second. This is what happens when you start on adrenaline. It is how most bad marathons are run.
• Even split — both halves at the same pace. Efficient and simple, and the target for most runners.
• Negative split — second half slightly faster than the first. Nearly every marathon world record has been run this way, and it’s the safest strategy for amateurs too.

The mechanism is physiological. Starting 10–15 seconds per kilometre slower than goal pace keeps you in a lower heart-rate and lower blood-lactate zone early, spares glycogen, and delays the “wall” — the point where muscle glycogen runs low and pace craters. Bank effort, not time. To know which zones you should actually sit in for those early kilometres, the Training Zones calculator turns your threshold heart rate or pace into Z1–Z5 bands; marathon effort for most runners lives at the top of Z2 into low Z3, not the redline.

Building a per-kilometre plan

Once you’ve set a realistic goal time, turn it into splits you can check on your watch:

1. Set goal pace. Divide goal time by 42.195 km. A 4:00 marathon is 5:41 per km (9:09 per mile). The pace calculator shows both units and a full split table so you don’t do this maths at 6 a.m.
2. Plan the first 5–10 km at goal pace + 8–12 sec/km. Deliberately, boringly slow. Let people pass you.
3. Settle into goal pace through halfway. By now the early crowd-surge runners are coming back to you.
4. From 30 km, hold or gently lift. If you have anything left, this is where a negative split is made — pick off the runners who started too fast.

Print or screenshot the split table and know your cumulative time at each 5 km marker. Chasing a cumulative target (“I should be at 1:59:30 by halfway”) is far more robust than chasing instantaneous pace, which jumps around with GPS noise, hills and water stops.

Don’t forget the course and the day

A pace plan is set on flat, calm assumptions. Adjust for reality:

• Hills cost more on the climbs than they give back on the descents — run them by effort, not pace.
• Heat is brutal over marathon distance; above ~20°C, add time and drink to thirst rather than forcing your original target.
• Wind on an out-and-back is not symmetric; the headwind half hurts more than the tailwind half helps — the same asymmetry cyclists model in the Cycling Power to Speed calculator.

The bottom line

Predict your ceiling from a recent race with the Running Pace calculator, then set a target a few percent slower if your mileage is modest. Run the first half controlled, aim for even-to-negative splits, and check cumulative time at each 5 km rather than obsessing over the instantaneous number. Do that and the last 10 km — the part that breaks most marathons — becomes the part where you pass people instead of being passed.