For the runners' high. Apparently,I am not alone in wondering about the reality of this phenomenon. According to this article in Tuesday's Washington Post, the "endorphin rush" was first identified in the 1970s in research done on marathon runners' blood levels. Later studies, however, showed that "endorphins don't cross into the brain easily. They get metabolized." On the other hand, some studies have demonstrated that exercise increases the levels of three key neurotransmitters, which are associated with elevated moods. On the other other hand, research has shown that just "walking for 30 minutes had a significant effect in depressed patients." But that was self-reported, so not conclusive.
Whatever. I can say that I'm off the antidepressant meds I was on for so many years, and I can directly attribute that to running. But that too is anecdotal, not scientific, evidence.
Even so, the "runners' high" eludes me. (Except when I set a new PR!)