I think you can do 100 miles on an ill-fitting bike without things going too far south. That becomes less true quickly as the distance increases. I think dealing with comfort on the bike is as important as any other aspect of preparation.
Good advice on nutrition. I'll just add, it's easy to get caught up in other things and fall behind on hydration and nutrition. Saddle or hand discomfort, socializing with others, chasing people, pacelineing, dealing with crappy weather, navigating; these things and others can take your mind off of nutrition. Even for a veteran. Things go very bad very quickly. I have to remember: if my attitude starts getting bad, that's an early sign I need to eat. The problem is, with a bad attitude it's hard to spot a bad attitude.