I've been reading John Knowles's classic, A Separate Peace, and ran across a terrific description of the runner's high. (Not that I've ever experienced it quite like this.) The narrator, Gene, prompted by his friend Finny, has taken up running. On a chilly winter morning they head out, Gene to run, Finny acting as his self-appointed coach. Gene has the kind of breakthrough that runners long for.
After making two circuits of the walk every trace of energy was as usual completely used up, and as I drove myself on all my scattered aches found their usual way to a profound seat of pain in my side. My lungs as usual were fed up with all this work, and from now on would only go rackingly through the motions. My knees were boneless again, ready any minute to let my lower legs telescope up into the thighs. My head felt as though different sections of the cranium were grinding into each other.
Then, for no reason at all, I felt magnificent. It was as though my body until that instant had simply been lazy, as though the aches and exhaustion were all imagined, created from nothing in order to keep me from truly exerting myself. Now my body seemed at last to say, "Well, if you must have it, here!" and an accession of strength came flooding through me. Buoyed up, I forgot my usual feeling of routine self-pity when working out, I lost myself, oppressed mind along with aching body; all entanglements were shed, I broke into the clear.
Wow! That's what I want! Lately I'm usually stuck in the "feeling of routine self-pity."