Give any two of pace, time and distance and this works out the third, in minutes per kilometre and per mile. It also predicts what the same fitness should produce over the standard race distances, and prints a split table you can read off during the race.
Riegel's formula: T2 = T1 x (D2/D1)^1.06. The exponent slightly above 1 encodes that pace inevitably degrades over longer distances. It is accurate within a few percent between 5 km and the half marathon, and gets optimistic for the marathon unless you have specifically trained the distance.
Most of it should be comfortably conversational, considerably slower than race pace - the widely cited split is around 80% easy, 20% hard. Running every session at a moderate effort is the most common way to plateau, because it is too hard to recover from and too easy to drive adaptation.