Where in the World is that Runner's High?

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!