Answer:
I defer to a friend of mine, Gerry Purdy, who allowed me to use his
"predicting race performance" tables in my books.

This allows you to run a few 5K races during your training program on
non-long-run weekends. Take the time and see what the equivalent
performance would be in the race of choice. Be sure to add a "fudge factor"
of 10-15 min in the marathon, and half of that for the half marathon (to
adjust for less than perfect conditions).

The table prediction assumes that you have done the long runs and other
training necessary for your goal, and that the race day temperature is not
significantly above 55 degrees F.

[MG editor's note: also check the race predictor calculator on the running calculators page]

- Jeff Galloway

Olympian and RUNNER'S WORLD columnist Jeff Galloway has helped over 100,000 runners in his clinics and training programs. For more running information you may see his books Galloway's Book on Running (2nd ed.) and Marathon: You Can Do It. Jeff conducts one day running schools (Chicago Aug 16, NYC Sep 6), Florida Beach Retreats (Aug 1-3 & Sep 12-14), and offers more info on his website: www.RunInjuryFree.com