I have used and written about the McMillan Running Calculator several times. A quick Google search of my posts mentioning McMillan returns 13.. In my experience, for stand alone foot race predictions it has been spot on. Now it has been updated with the following:
You asked and we listened. High school coaches wanted the 1600 and 3200 meter races added to the Calculator so in the most recent update, we've added them. Cross country runners and coaches wanted 6K and 12K added so we've added those as well. Military, police, fire and other emergency personnel asked for the 1.5 mile distance that they often have to use as a performance test and ultra runners wanted the 50K, 50 Miles, 100K and 100 Miles so we added them as well (though ultra runs are very hard to predict due to the variation in terrain and environmental conditions).
You'll also notice that we've widened the Recovery Run, Long Run and Easy Run pace ranges. We found that runners who wear speed/distance monitors were forcing themselves to jump right into the pace ranges whereas those who ran more by effort eased into the paces. So, we widened the range to better match what runners should do - start easy and gradually pick up the pace. Research and practical experience also taught us that while runners at the front of the pack did their easy runs slower than marathon pace, runners at the middle and back of the pack needed to spend more time at around marathon pace (or even slightly faster) so some runners will notice that their Endurance Workout paces should be a little faster.
Does the Calculator work for you?