Pace is time per unit distance; speed is distance per unit time. Runners think in pace because it is what a watch shows and what you hold during a race. They are reciprocals of each other.
Race distances do not scale linearly
Doubling the distance costs more than double the time, because you cannot hold the same intensity. A common rule of thumb adds roughly 15 to 20 seconds per kilometre when moving from 10 km to half marathon pace, and a similar amount again to marathon pace. Riegel’s formula makes this precise with an exponent of about 1.06.
Even splits versus positive splits
Most personal bests over longer distances come from even or slightly negative splits — running the second half at the same speed or marginally faster. Starting too fast is the most common pacing mistake, and the time lost in the final quarter almost always exceeds the time gained early.
Converting between units
A mile is 1.609 km, so mile pace is always the larger number. 5:00/km is 8:03/mi; 4:00/km is 6:26/mi. A four-hour marathon is 5:41/km, and a sub-three-hour marathon needs 4:16/km sustained for the full 42.195 km.