You lose yourself in the run (or sometimes fast paced walk).
You lose the anxiety.
You lose the pain.
You lose the feeling on edge. It releases endorphines. It provides me with reassurance that even though I 'think' I'm having heart problems instead of my gastritis, that reason can help intervene when I'm having attacks. I'll say to myself, "Ok, genius, if you're having a heart attack, how in the world did you just run 1+ miles, and felt great the whole time?