On pure pedantry the author discusses an article in which the runners high is shown with imaging.
The authors of the article discussed measured the resting endorphin activity in the brains of 10 athletes using PET scans. They then sent the runners out on a 2 hr run. After they returned they put them back in the scanner and looked at endorphin activity again. They compared the images before and after the run to look for what areas of the brain had greater endorphin activity.
This result provides further evidence that the runner's high is caused by endogenous opioid release in the brain. What is interesting to me is that you see similar brain activation for a variety of different types of rewarding events -- whether they be drugs or video games or anything. I seem to remember that they even showed that in academics these parts of the brain are activated when they learn! This similarity of reward activation in a variety of behavioral contexts implies two things: a common system for analyzing rewards and a wide variety of things that humans have found to activate this system. It would appear that it really is "whatever floats your boat" that you find rewarding.
The results and details are presented in this post on pure pedantry
Related post on this blog: What do addiction and running have in common