Well... the THC is stored in the fat of the body, so when you burn fat (during physical exercises, or fasting) the THC is released again, making you high.
I'm almost certain the THC stored in your fat has been hydrolyzed and is in an inactive form. Burning the fat will not reactivate it. If that were true, you would get high every time you work out or break a sweat for a while. I know runners high is a relatively rare thing for most people (I've only had it twice and ive been working out for many years)...

Those chemicals that cause a runners high are endogenous opioids, not cannabinoids. The opiods are things like enkephalins, endorphins, and others...