Decide well in advance

Decide before the race the conditions that will cause you to stop and drop out. You don’t want to be out there, saying “Well gee, my legs hurts, I’m a little dehydrated, I’m sleepy, I’m tired and it’s cold and windy.” And talk yourself into quitting.

If you are making a decision based on how you feel at the moment, you are most probably making the wrong decision.