Pace: time per unit of distance
Pace answers "how long does each kilometer take?" — total time divided by total distance. For a 10km run completed in 50 minutes: 50 ÷ 10 = 5 minutes per km, written as 5:00.
Speed: distance per unit of time
Speed flips the question around: "how far in an hour, at this rate?" — distance divided by time, scaled to an hour. The same 10km-in-50-minutes run: (10 km ÷ 50 min) × 60 = 12 km/h.
The same pace, a different distance
A 5km run completed in 25 minutes gives the exact same pace: 25 ÷ 5 = 5:00 per km, and the same 12 km/h speed. Distance and time both halved, but their ratio — the pace — stayed identical, because pace measures the rate of the run, not its length.
Why both figures are useful
Pace is the figure runners typically track and compare against a goal ("I want to run sub-5:00 pace"), since running culture is built around per-kilometer or per-mile splits. Speed is more familiar for comparing against non-running contexts, like a vehicle's speed limit, or for sports that quote performance in distance-per-hour rather than time-per-distance.