It usually happens to a greater degree during the run. Feeling an almost out of body experience where you can run as hard as you want and not have pain. It is almost like watching yourself in a fluid state of motion; little effort required; everything seems so easy and almost dreamstate. Commonly referred to as "Runners high'.
Endorphins are naturally produced morphine like drug that the body produces. Fun stuff.
Paul http://community.active.com/blogs/p-dog
