I agree with Maillotpois. There are so many things that you need to consider when planning such a long ride. And at least 50% of it is mental. I've only done one century (4 years ago), with pretty much no training, other than the normal riding I do, which is riding lots of hills. While this century is billed as one of the easiest on the east coast, what they don't tell you is that on the return trip, there is a vicious headwind, as it's by the coast. Add in a torrential downpour and 20 degree temperature drop at mile 85, it was a recipe for disaster. But, I knew I could do it, and I wasn't going to quit. Sure I felt a little out of it that night, but I have had the same feeling after a hard 40 mile ride.
So plan, prepare, and ride, but don't over analyze. You'll either do it or you won't, and even if you don't finish, you will have achieved something pretty awesome.
2015 Trek Silque SSL
Specialized Oura
2011 Guru Praemio
Specialized Oura
2017 Specialized Ariel Sport