Pace and speed are inverses
Pace is minutes per distance; speed is distance per hour. Runners think in pace because it is what a watch shows and what a race plan is written in — "5:00 per kilometre" is directly actionable in a way that "12 km/h" is not.
They move in opposite directions, which is the source of most confusion: a faster run has a lower pace number and a higher speed.
Even splits, and why almost nobody runs them
Nearly every marathon personal best is run with the second half at the same speed or slightly faster than the first. Nearly every disaster starts with a first half run too fast, because the early kilometres feel easy.
Working out the pace beforehand and holding it — even when it feels absurdly comfortable at 5km — is the single most reliable improvement available to most amateur runners.
Race distances are not round numbers
A marathon is 42.195 km, a half 21.0975. Planning against 42 km leaves 195 metres unaccounted for — about a minute at a four-hour pace, which is enough to miss a target. The exact distances are used here.
Frequently asked questions
What is a good pace?
Entirely relative to you. A useful reference is that a conversational pace — one you could talk at — is where most training should sit, whatever the number.
How do I convert pace to speed?
Divide 60 by the pace in minutes. A 5:00/km pace is 12 km/h. They move in opposite directions, which is what trips people up.
Should I plan even splits?
Yes for most distances. The common failure is a first half run too fast, because it feels easy at the time and is paid for later.
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