Pace is time per unit distance; speed is distance per unit time. Runners think in pace — minutes per kilometre or per mile — because that is what a running watch displays and what athletes actually hold steady during a race, whereas cyclists and drivers more naturally think in speed. The two are simply reciprocals of each other, and converting between them is one division either way.
Race distances do not scale linearly with time
Doubling the race distance costs meaningfully more than double the finishing time, because sustaining a given intensity gets progressively harder as duration increases — the physiological systems that fuel a 20-minute effort are not the same ones that dominate a 3-hour effort, and fatigue, fuelling and pacing discipline all become bigger factors as distance grows. A common rule of thumb among distance runners is to add roughly 15 to 20 seconds per kilometre when moving from 10 km race pace to half marathon pace, and to add a comparable amount again when stepping up from half marathon to full marathon pace. Riegel’s formula, a widely used race-prediction model in running, makes this relationship mathematically precise using a fatigue exponent of approximately 1.06, applied to the ratio of the two distances — a small-looking exponent that nonetheless produces the substantial pace slowdown experienced at longer distances.
Even splits beat positive splits, almost always
A negative split means running the second half of a race faster than the first; a positive split means slowing down in the second half. The overwhelming pattern among personal-best performances at longer distances is even or slightly negative splitting — holding the same pace throughout, or running marginally faster in the closing stages. Starting too fast is the single most common pacing mistake made by both beginner and experienced runners alike, and the time lost struggling through the final quarter of a race from starting too aggressively almost always exceeds whatever time was gained by running the opening kilometres faster than planned. This is why experienced coaches consistently emphasise conservative starts even when a runner feels deceptively fresh in the first few kilometres.
Converting between kilometre and mile pace
A mile is 1.609344 kilometres, so a mile-pace figure is always the larger number when compared to the equivalent kilometre pace, since each mile covers more ground than each kilometre and therefore takes longer to run at the same underlying speed. A pace of 5:00 per kilometre works out to 8:03 per mile; 4:00 per kilometre is 6:26 per mile. Reference points worth knowing: a four-hour marathon finish requires an average pace of 5:41 per kilometre sustained for the entire distance, while breaking three hours — a serious benchmark for recreational marathon runners — requires holding 4:16 per kilometre for the full 42.195 km, an average that has to survive fatigue, terrain, and weather across more than two and a half hours of continuous effort.
Why split times matter more than average pace alone
Two runners can finish a marathon in the identical time while experiencing very different races — one holding a smooth, even pace throughout, the other starting fast and fading badly in the final 10 kilometres. Average pace, calculated simply as total time divided by total distance, cannot distinguish between these two very different pacing strategies. This is why serious runners track kilometre or mile splits during a race rather than relying on the average pace alone — the split pattern reveals whether the effort was well judged or whether an earlier, faster start quietly borrowed against a slower, harder finish.