In the 1970’s, counter-culture types in the mountains north of San Francisco took to careering down Mount Tamalpais. They were riding for adventure, for exploration, and as a way to interact with the landscape; they were not riding for exercise.
Check out this History-of-Mountain-Biking-101 from Sarah McCullough, whose PhD dissertation at UC Davis explores the history of mountain biking and explains how this group of renegade cylists invented the sport.
Renegades of Bike Culture | Fig. 1 from University of California on Vimeo.