Without a doubt, the easiest way to avoid a crash after the post-race high is to pick a new goal. Decide what you want to tackle next -- whether it's getting faster, going further, being more consistent with your training, trying complimentary workouts, or maybe even setting your sights on a different sport altogether -- and put a plan together for how you'll go about achieving it.
Long distance runners are a bizarre group of people to those who find comfort in stillness, pleasure in overindulgence, and reassurance in conformity. I know this because I was not always a runner. There was a time that I scoffed at runners and the strange counterculture of running. But, now I know better.