Spring is finally here, and if the weather looks good, many of us will head outside this weekend to stretch our legs. Some will content themselves with leisurely strolls in the park while those looking for more of a challenge may prefer to chase that legendary, though misunderstood, “runner’s high”—the heightened sense of well-being and euphoria that endurance running can produce.

Scientists used to chalk these feelings up to an increased concentration of endorphins, but we now know that the runner’s high comes from the so-called...