Runner's high: a feeling of euphoria that is experienced by some individuals engaged in strenuous running and that is held to be associated with a release of endorphins by the brain.
I guess the operative word in that definition is "some." I'm not experiencing anything even remotely resembling a state of euphoria. All I feel is utter exhaustion -- and it isn't getting better as the days go by. I'm starting to fear that the purported "high" has an expiration date...and I left it behind decades ago!