Well, move over endorphins--you don't get all the credit anymore. The real chemical hero of the runner's high? Endocannabinoids. If this term sounds suspiciously like cannabis, that's because endocannabinoids are basically chemicals similar to marijuana, only they're produced naturally by the body.
If you're more interested in the topic, the blog post, "Honing in on the Source of the Runner's High" from the New York Times gives a pretty good explanation of the science behind endocannabinoids and endorphins, and how the runner's high could be produced. Another interesting article from NPR called, "How Runner's Get High" also does a nice job of explaining that elusive feeling of euphoria sometimes experienced while running.
Some runners don't wait for a natural high. Recently, I've seen more and more articles about running under the influence of marijuana. While this may sound counter intuitive, knowing how a runner's high is produced, it might make more sense. Weed could be acting as a shortcut to get a runner's high, and may enable some athletes to reduce their pain perception and run longer and harder.
Below are a couple interesting articles about running under the influence of marijuana. Please don't take this as a recommendation--running after getting high is definitely not something I'm thinking of doing.
"The Health Benefits of Running While High"
"Can Marijuana Make You a Better Runner?"
Again, don't take these articles as a recommendation from me. I'll continue to pursue my runner's high through my body's natural endocannabinoids--not from smoking dope. But to each their own.