If you think that 8 hours a day, 6 days a week was bad, you should consider early bike races. The book Bike Cult describes some early bicyle races (late 19th century or early 20th century?) done on an indoor track. Racers tried to do the most laps they could for up to 20 hours a day for 6 days. If the book is describing these races accurately, it must have been fairly brutal on the body -- possibly even more brutal than the bare-fisted boxing events from the early days of boxing.

This type of racing didn't last long. They soon modified this race so that they racing was done in a tag team fashion, so that one team member would rest while the other team member would race. And the idea of the 20 hour racing days didn't last too long.