How pace is calculated
Pace is simply total time divided by distance: running 10 km in 52:30 gives a pace of 5:15 per km. Speed is the inverse — 11.4 km/h in that example. The race predictions use the Riegel formula, T2 = T1 × (D2 / D1)1.06, which models how fatigue slows you as distance grows.
Using the predictions
Riegel assumes your input effort is a genuine race-pace effort and that you are similarly trained for the longer distance. If your 10 km was a casual jog, the marathon prediction will be optimistic — and most runners find the half marathon prediction the most reliable of the four.