There is direct evidence that endorphins are released following a period of strenuous exercise. What it takes for endorphins to be released is unclear (and probably varies from person to person), but the consensus seems to be that the body has to cross over some threshold of strain before endorphins are released. I’ve certainly experienced this myself (I think). The first two miles or so of my recent runs are usually pretty tedious and unpleasant. I get tired and want to stop. Sometimes I get side pains. But then I start feeling better and am able to run another three miles without significantly slowing down. I’ve never gotten side pains during the second part of a long run (and it’s not because I selectively stop running). So I do think the time-delayed endorphin release is real.
Which brings me to the million dollar question – is there a way to get the endorphins to be released right before a run, to spare the body the pain of the pre-threshold strain? Here’s my X million dollar idea (where X>=0): use classical conditioning, the idea behind Pavlov’s dog, to train the body to release endorphins. Have someone run for a while. Once the subject experiences an endorphin release, play a particular song. Eventually, the body will be trained to associate the song with releasing endorphins. Play the song right before you’re ready to run and voila! Instant endorphin-filled run!
Seriously, I wish someone would do this experiment and see if it works. I may try it on myself and report back.