Note: this program is based on Jack Daniels vdot formulas, as described
in "Daniels Running Formula" and made more explicit in "Oxygen Power".
The predictor works quite well for typical road race distances - 5k up
to half marathon. However, it is somewhat less accurate for predicting
marathons, because the assumptions behind the formula come unstuck
for races of substantially longer than 2 hours duration.
Another issue is
that the formula doesn't take account of individual variation.
Use common sense here -- no formula can accurately predict half-marathon times
from 800m times. The best way to use prediction formulas is to stay within
a ratio of 2 or less for the two distances.