> I have never felt "pure pleasure" during a run.I've often have pure pleasure while running through nature if you count that. I've done a few long distance trail events over the years, and it seems rather good for my mental health.
I've only done the pure joy of running thing, the so called runners high, once or twice. That was while doing fast⁰ road¹ running before I discovered the joy of a slower long plod in the green stuff, and it literally feels like being a little drugged up. The physical joy of exercise usually comes from neuroactive chemicals that the brain kicks out as natural painkillers², when you finish these take a while to filter out of your system but no longer have effect of hard work to counteract, so you feel really good in that cool-down period. Sometimes though their levels end up a little higher than they need to be for your current efforts while still running, when that happens it feels like you are running on clouds and that you could keep going for days, if you notice that happening you can sometimes draw it out by careful management of your level of effort (but obviously your judgement in that regard is a bit off at the time so that is easier said than done).
----
[0] well, fast-sh, I've never been an example of peak human fitness!
[1] or at least payment, or the cycle tracks that were a large part of my regular training runs
[2] a useful part of the flight/fight reaction when being hunted, which as persistence hunters we evolved to co-opt when on the other side of that great survival game