Peter Riegel did basic formula for predicting race time. The formula is really simple as it essentially states that roughly speaking, a person's speed declines by around 6% when the distance doubles.
Peter Riegel's formula is: t2 = t1 * (d2 / d1)^1.06.
Peters work is relevent to most sports so it is can be applied to swimming, cycling and running.
If you put in a distance and time you have done the calculator will predicate the times you will be able to achieve for other distances.