There is some thinking that very positive rewards, particularly natural rewards (in this case, they have been addressing sexual behavior), may have a protective effect on drug addictions. In other words, someone who has another positive experience (reward) prior to trying drugs may not be as easily addicted as someone who is not getting these positive rewards. I hope I've explained that clearly. It seems to me that a runner's high may fall into this category of natural rewards. Drugs of abuse act through similar pathways as natural rewards, but something remains different that makes them truly addictive. There are people working on understanding these differences as well. If we can understand what is different between a reward induced by a natural behavior than a reward induced by a drug of abuse, we may find some insights to help us treat addiction.
But, like I said, this is still untested, so don't take this as fact.