Didn't know about the runner connection. Back in the '70s, "long slow distance," a.k.a. LSD, was a term used in the bike racing community for a then-new training concept. As I remember it, the idea was that the best way for a racer to lose weight was by going out on the bike for as many hours as you could stand while riding at a minimal effort level, the iron rule being that you must avoid so much as breaking a sweat on the ride.

The term was later quietly amended to "long steady distance."