The classic mistake: starting too fast
Starting gun, music, cheering crowds: the first kilometres of a race almost always feel easy. That is exactly why so many runners start too fast. It takes its toll later. If you run beyond your means in the first part, fatigue builds up faster and you lose more time at the end than you gained at the start.
An example over 10 km: you run the first 5 km at 4:50 min/km instead of the planned 5:00 and then have to drop back to 5:15 min/km. Your finish time is 50:25 instead of 50:00. The fast start did not gain you time; it cost you time.
What is a negative split?
A negative split means running the second half of a race faster than the first. With an even split both halves are equally fast; with a positive split the second half is slower. Many coaches recommend an even or slightly negative split, because that is how you spread your energy best.
The key word is “slightly”. A negative split does not mean dawdling through the first half and then sprinting. A difference of one to three percent is enough in most cases.
In record runs at elite level you often see almost perfectly even or slightly negative splits. For recreational runners, even an even split is a big step forward, because in practice many races end with a clearly slower second half.
Worked examples
With a negative split the average pace stays the same. The first half gets a little slower, the second a little faster. Some examples with a 2% negative split:
| Goal | Avg. pace | 1st half | 2nd half |
|---|---|---|---|
| 10K in 50:00 | 5:00 min/km | 25:15 (5:03 min/km) | 24:44 (4:57 min/km) |
| Half marathon in 2:00:00 | 5:41 min/km | 1:00:36 (5:45 min/km) | 59:23 (5:38 min/km) |
| Marathon in 4:00:00 | 5:41 min/km | 2:01:12 (5:45 min/km) | 1:58:47 (5:38 min/km) |
So the difference between the halves is only about six to seven seconds per kilometre. The first half feels pleasantly controlled, and you have reserves left to pick up the pace at the end.
Planning your splits
For the plan to work, you need split times. In a 4:00:00 marathon with a 2% negative split you should be at roughly these marks:
1. 10 km: 57:27
2. 20 km: 1:54:54
3. Halfway mark: 2:01:12
4. 30 km: 2:51:20
5. 40 km: 3:47:38
The pace calculator builds this split table for any distance and target time. Just set the negative split slider to the percentage you want. Many runners write the times for every fifth kilometre on their forearm or a wristband.
Tips for putting it into practice
1. Deliberately hold back for the first two kilometres. If it feels too easy, it is usually right.
2. Watch averages, not individual kilometres. A kilometre uphill or in a crowd may be slower.
3. Only speed up after halfway. And even then only as much as feels good.
4. Practise in training. Occasionally run your long runs with a slightly faster second half. That way you get a feel for it.
A negative split is a tool, not an obligation. Heat, wind or a hilly profile can make a different approach more sensible. Which finish time is realistic in the first place is covered in Predicting Your Marathon Time.