They mashed the hell out of the saddle, and occasionally walked. You also didn't really see some of the stupid steep stuff like the Angrilu or Mortorolo, and if they did have hills like that, a lot of riders might have walked them (many of the riders reported that basically everyone walked or got pushed up the Muro Di Sormano when it was put in the Tour of Lombardy in the 60s briefly)
Training was much different back then too. Remember hearing about stuff where your DS would put your bike in the big ring, and send you on a ride. When you got back, he'd go over your little ring with a microscope to make sure there wasn't a spot of chain grease on it. They were not training bike racers to spin up hills back then.