Brandy,

Given the shape you are already in... I'd say if you can ride 40-50 miles a couple of times in the same week that'll give you a good base. I'd be doing at least 80-100 miles a week with one long ride for at least several weeks ahead of the ride and you'll be just fine!!! Plus a few hilly rides just so you get the hang of the granny gear (cause you can't power up the hills that are several miles long all the time! )

I'm working on base still but will be doing mostly 25-50 milers with a metric and a 70 miler thown in. I'm still slow as molasses though!!!

Trac'

ps. You'll probably get some better advise from some of the riders that have more experience but you are already really strong and probably just need to learn how to pace yourself for that distance!