“Here’s a quote from ultramarathoner Dick Collins: 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 leg 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 that moment, you will probably make the wrong decision.”
Seth Godin, The Dip: The extraordinary benefits of knowing when to quit.