Well, I can't claim "so often". I don't ride 100+ so much, but I do get to do 70+ pretty often. I like riding solo, which usually means unsupported too. Riding with someone, especially in a paceline for increased speed, means not being able to see what's around you, not being able to slow or speed up as you prefer, not being able to stop any old time as the whim strikes.

As some here said, riding a long distance requires you know yourself. You must eat enough and it must be the right stuff. You must drink enough water and be wary of salt depletion on a hot day. You must know how hard to push so that you don't wear yourself out unnecessarily and yet go fast enough that the trip won't take foreverrrrr.

When you finish you feel great!