There is no question there are measurable mood changes after extended endurance exercise. In fact, researchers have shown a positive change in mood among clinically depressed subjects following extended exercise. The real issue is what causes it. The endorphin role has been debunked. The latest research shows that the real cause of the "runner’s high" is the endocannabinoid system.
The endocannabinoid system is a series of transmitters and receptors spread throughout your central nervous system. The endocannabinoid system plays an important role in reward, learning, appetite and mood control. Whereas endorphins act on the body's natural opioid receptors, endocannabinoids act on the body's natural marijuana receptors, specifically THC, the psychoactive component in marijuana. Endocannabinoids are small molecules that can easily travel to the brain. Most importantly, extended exercise increases the levels of circulating endocannabinoids.
Endocannabinoid release is dependent on exercise intensity. High intensity as well as very low intensity exercise such as walking does not increase endocannabinoid release. Moderate intensity exercise for extended periods, produces the greatest amount of endocannabinoid release. Although you may not have heard of endocannabinoids, you probably have heard of CBD. CBD acts on the endocannabinoid system which may explain its diversity of effects.
Further research will better define the role of the endocannabinoid system and the runner's high but we should, at least, change our terminology. We can no longer say "endorphin high" but "endocannabinoid high" doesn't have the same ring. Perhaps "canna high".