Yeah its a strong chemical, I saw some runners mentioned he even could finish running a marathon with a stress fracture (he did not know he had it)... of course he could barely walk after the effect wears out..
Sometimes its because of endorphin, sometimes not.. But better run without pain. The motto "No pain no gain" is out. It should be "Pain is no gain".
If we can get our 10K, HM or FM in our PB all the depression will left away.
Therefore, we should careful to set our Goal Time, don't set it too high if we haven't enough time in training for the even.