Yep, gearing is a big part of the picture here. Back in my race days I ran a 42/52 up front and a 12-21 6spd out back and did regular training rides of 100 miles or more on it. Sure, the hills were killer events, however I just grunted through it along with everyone else.
I'm 65 now and can do 100 miles without too much trouble with a low 34 up front and 32 out back. On the touring rigs it is even lower gearing than that and it makes 100 miles very doable.