There is one very important point the fellow noders have missed. Runner's high is not only about the chemicals released in a body. Runner's high is no doubt boosted when you finish a race or a training by the good feeling and self-respect. Running doesn't have that kind of social stigma as, say, marijuana or other substances have. Even the police thinks it's okay to run. A post-race euphoric state is really well-deserved because you have done good job and whereas smoking pot spoils your lungs, running is an investment on the health. I think my runner friend put it right when he said that you always have to pay a price to enjoy something definitely. The exhaustion caused by running is the price but the award is twice as great.