Wiki tells that a beta endorphin is an endogenous opioid peptide neurotransmitter. Key in on the word, opioid, as in opium. Just like morphine and codeine, it is addictive. A "runner's high" is when the body stressed sufficiently to produce endorphins. But it has to be from a long, sustained, workout. Something that I didn't know - the time of the endorphin release coincides when the body had used up its store of glycogen. Some have considered the "runner's high" as a myth, but brain scans have confirmed what many a dedicated runner can tell you - it feels good.
Runner's High describes the feeling as euphoric, or a sense of invincibility. Maybe it is because the body has depleted its reserves and it is the will of the mind that keeps the runner moving. The body and the mind have disassociated, and free of the body, the mind feels freedom. Perhaps I am addicted to it, because when I cannot run because of an injury or really bad weather, I miss my morning 'buzz' from running.
So how did we get here? Minor Variations in One Gene May Be Associated With Endurance Running explains that endurance athletes are likely to have variations of the NRF2 gene. The Science Daily article explains that the gene is suspected to a) produce additional mitochondria - the power centers of living cells, and b) reduce the dangers of oxidation and inflammation. Interaction between SNPs in the NRF2 gene and elite endurance performance explains that sprinters lack the gene, but the correlation is extremely high for long distance runners.
It is interesting isn't it? How marvelous our bodies are!