Short answer: 5,000 m in 1,500 s is 3.333 m/s, which is 12 km/h — a pace of 5 minutes per kilometre. Enter a speed of 10 m/s over 60 s for a distance of 600 m.
Keeping the units straight
The page works in SI units internally: metres and seconds. km/h and minutes per kilometre are then computed for reading, so a conversion is never a step you can skip by accident. That is the mistake behind most "my speed is wrong" complaints.
Worked example
A train covers 240 km in 3 hours: 80 km/h, which is 22.22 m/s. In the page, enter 240,000 m and 10,800 s — the same journey expressed in SI — and the derived km/h row confirms the 80 you expect.