Just some thoughts about a century:
It is a long ride. Anything over a metric (62 miles) is a long ride. The body starts to break down after certain distances. I can easily do 80 miles but after that, I struggle (no matter how fit) with booty problems, sore feet and a stiff neck and arms.
I don't feel you have to do a century to do a century. In other words, if you sign up for an organized century ride, it's not necessary to do 100 miles before that. BUT you must have a good base. You should be riding or spinning a certain number of miles each week. I would say doing at least 20 - 40 3 - 4 times a week. You should be able to do a long ride on the weekend (at least 60 miles). There are alot of good articles on training for a century.
Study up on nutrition. I feel a century is 20% physical preparation, 60% mental and 20% nutrition. Poor nutrition, poor ride. Leads to bonking.
Can you ride through pain? If not, learn how.
Fine a large century to try. The more people on the ride, and more physical contact with other riders, the more motivation to continue. I wouldn't pick the hilliest century around. Keep elevation gain to around 3500' or less.
Never worry, there is always a sag. And another time. Do you best and enjoy.
PS: I think something other than a road bike makes the ride more difficult. A hybrid bike is heavy and not particularly comfortable for long distances but you will know what's best. Plus I'm a large woman (5'8", 200 #s) and have no problem on a regular road bike. I also do centuries. Don't have to be a little, skinny person to be fit and do well. Climbing is tougher - no way around that, but it's doable.