Found a nice article about the Runner's High (thanks RunLawrence). Funny thing was that I was just saying to a friend last week how I never seemed to "feel it" until I ran at least 5 or 6 miles. That goes right along with what the article says.
In a groundbreaking 2003 experiment, scientists at the Georgia Institute of Technology found that 50 minutes of hard running on a treadmill or riding a stationary bicycle significantly increased blood levels of endocannabinoid molecules in a group of college students.I guess that makes perfect sense to me now. I wondered why I never felt that way after running a 5k on the treadmill or on the road but seem to feel it after running the trails or a long run on the weekends.