Your first century will be an exersize in pain management and fatigue management. You need to spend a lot of time on the bike getting used to sore neck and sore butt, and you need to learn how to pace yourself so that you'll have plenty in the tank to finish.
The mileage isn't really that hard, but it will be a big accomplishment.
You should probably plan on about 6-1/2 hours ride time, and 8 hours total time.