Building a Marathon Pace Plan: Splits, Negative Splits and Race Predictions

Start with the arithmetic of the goal

A marathon is 42.195 kilometres, or 26.219 miles. To find your average pace, divide the goal time by the distance. For a four-hour marathon that is 14,400 seconds divided by 26.219 miles, which is 549 seconds per mile, or 9:09. In kilometres, 14,400 divided by 42.195 is 341 seconds, or 5:41 per kilometre. Here are other common goals worked the same way:

• 3:30: 8:01 per mile, 4:59 per kilometre
• 4:00: 9:09 per mile, 5:41 per kilometre
• 4:30: 10:18 per mile, 6:24 per kilometre
• 5:00: 11:27 per mile, 7:07 per kilometre

Because pace per kilometre and pace per mile are different clocks, pick the unit your race uses on its course markers, and keep your watch on that unit. Converting in your head at mile 20 is a bad use of brainpower. If you ever need to convert, the pace calculator and the unit conversion tool will do it before you leave home.

A pacing table for 4:00:00 at even effort

Even pace means the same speed throughout. For a four-hour finish, the checkpoints in kilometres look like this:

• 5K: 28:26
• 10K: 56:53
• Half marathon (21.1K): 2:00:00
• 30K: 2:50:38
• 35K: 3:19:05
• 40K: 3:47:31
• Finish: 4:00:00

Print this on a wristband or save it as a note. A table beats mental math in the final ten kilometres, when most people cannot add reliably. If you are slightly ahead at 30K, you will not feel any wiser than if you are slightly behind. The table does the thinking.

Even, negative, or positive: what splits actually look like

A split is the time for one segment, and the pattern of splits across a race matters as much as the average. For the four-hour goal, three patterns:

• Even split: 2:00 for the first half, 2:00 for the second, both at about 9:09 per mile.
• Negative split: first half in 2:01:30 (9:16 per mile), second half in 1:58:30 (9:02 per mile). You run the second half faster, finishing at the same 4:00.
• Positive split: first half in 1:55:00 (8:46 per mile), second half in 2:05:00 (9:32 per mile). The same total time on paper, but you run the second half 46 seconds per mile slower than the first.

All three finish in four hours. They feel completely different. Studies of race results regularly show that most runners slow in the second half, and that elite performances tend to be close to even or slightly negative. A positive split like the one above is the typical outcome of starting too fast: going out at 8:46 feels easy at mile 3 and costs a lot more than it feels like by mile 20. Starting slightly slower than goal pace, say about 5 to 10 seconds per mile, and speeding up in the final third is the usual advice.

Look at the cost of an apparently harmless mistake. Running the first ten miles at 8:59 instead of 9:09 saves only 100 seconds by mile 10. If it makes you slow by even 15 seconds per mile over the final ten miles, you lose 150 seconds and end up behind. The early gain is small. The late penalty is not.

Check the goal itself with the Riegel formula

Is four hours a reasonable target for you? A well-known estimate comes from Pete Riegel, who proposed in 1977 that race time grows with distance roughly according to a power of 1.06:

T2 = T1 x (D2 / D1)^1.06

T1 is a recent race time at distance D1, and T2 is the predicted time at distance D2. Say you ran a 10K in 50:00. The prediction for a half marathon is 50:00 x (21.0975 / 10)^1.06 = about 1:50:19. For a full marathon, it is about 3:50:01. A 25:00 5K predicts 52:07 for 10K, 1:55:00 for the half, and about 3:59:47 for the marathon.

Treat these numbers as ceilings, not promises. The formula assumes you have trained as thoroughly for the longer distance as for the shorter one. A runner with a fast 5K and short weekly mileage will usually finish a marathon well behind the prediction, because the marathon depends on endurance, fuel and durability that a 5K does not test. A reasonable approach is to use the prediction from a half marathon and add a margin, instead of predicting a marathon straight from a 5K.

Race-day rules that protect the plan

• Run the first 5K slower than it feels like you should. Adrenaline lies.
• Check your pace against the table at fixed points, not every minute. Constant watching causes overcorrection.
• GPS watches drift in tall-building areas and tunnels, and measured courses are usually walked on the shortest line. Expect your watch to read slightly long, so use mile or kilometre markers for the real pace.
• Plan fuel and fluids, since muscle energy stores can run low late in a marathon and cut pace regardless of fitness. Your own nutrition plan should be tested in training.
• Have a bad-day plan. If the first half feels hard, accept a slower goal early. Slowing by 10 seconds per mile at mile 8 costs far less than collapsing at mile 22.

This is general information for healthy runners. If you have a medical condition or have not trained for the distance, talk to a doctor before attempting a marathon and follow a structured training plan. Once you have a goal, enter your target time and distance into the pace calculator to get the per-mile and per-kilometre pace, and use the time duration calculator to add up split times.