Just putting my two-penn'orth....
Its a great thing to set yourself a goal like a century. You do get a real sense of acheivement when you finally cross that line. Its very weather and terraine dependent on how much you enjoy it as well as fast you do it!
Having moved to New Zealand for a year or so we wanted to do as many events as we could while we were here, the UK is not as blessed with big events.
We did the Taupo Challenge in coldish weather- max 12 degrees, wet and windy and very hilly, in 6 hours. Then we did the Taranaki Challenge - flatter, still day but very hot 31 degrees in 5 hours 17 mins. We loved the Taranaki and were just glad the Taupo event was over!
If you are trained and have done some big distances, rested in the week prior and take plently of nutrition and hydration on the way round you won't need to stop for a rest. In fact those that did had a hard time getting going again. (On Taupo some riders even got hypothermia when they stopped and got to finish in the Ambulance!)

Good luck with the training.