By Tanner Garrity Either cocktail napkins are an unlikely candidate for “survivorship bias” — they’ve reportedly given us Harry Potter, Pixar, Shark Week, the Super Bowl trophy and the first model of Chicago’s grid — or there really is some magic to scrawling an idea down at the bar at the end of a night. At some point in 1975, the same year Steve Prefontaine died, a first-year track coach at the University of Michigan named Ron Warhurst sat down for a few beers and got back up with a blueprint of the most notorious running workout ever created. Warhurst couldn’t stop thinking about a circuit …