The 'runner's high' is a mild euphoric state induced by long-duration repetitive exercise, caused by the body releasing endogenous opioids.

“Perhaps some of you have heard of the so-called runner's high. The runner's high is a euphoric state. It's a fairly mild euphoric state in most cases. But it's a euphoric state induced by long-duration effort, a.k.a. the runner's high. The runner's high is just a phrase used to explain that when we engage in long-duration repetitive action effort, the body starts releasing these endogenous opioids they bind to receptors, and they trigger things like analgesia, relief from pain. They trigger mild euphoria. They tend to change our perception of the outside world, make things look more beautiful, shiny. They give things a little bit of a sheen.”