Hi Mike, if you're talking about the Blood Sweat & Gears, you should definately start out with the 50 mile ride. There's also one in Ashe County called the Blue Ridge Brutal. It's pretty tough as well. There are several other century rides here, and a good one in Brevard, the Assault on the Carolinas.
as far as how much riding on flat land equals how many feet of climbing, I have no idea. My only recommendation would be ride ever hill in your area. Do hill repeats on them if you can. spend a weekend in the mountains riding the hills. On the flats, do sprints. Lots of sprints.
Oh, and the BSG 50 miler has 7500 ft. of climbing and the century has 13000 ft of climbing. That's a ton of climbing even if you've done it before.