You've been riding before, so you have to have some decent base level of fitness (probably a lot - a 12hr anything race would kill me and I ride 4K+ miles/year).
I would do what you can to build up some endurance, even if it's on the trainer (2 hrs at a time seems to be my limit and that is with serious motivation and peer pressure). Maybe a slightly lower pace - you're not going to be racing the century ride. Build up over say 3 weeks, back off a bit for a week, then build for remaining 2 weeks. Make whatever sacrifices to the weather dieities for early spring (in spite of P. Phil's prognostications) so you can get some time outdoors. Take it easier for the week of the ride, take it easy on the ride; concentrate on finishing rather than setting any records or burying your friends. Good luck!