Octoberblue,
Come to Southwestern Ohio, you'll get lots of hills here. I was riding the trail today and I was thinking about how hard flat terrain was. You just never get a break, you just pedal, pedal, pedal. With hills you can spin down, coast down, stand to get up, etc. There's so much more variety.
I used to hate hills but with a smaller, lighter bike I really enjoy them. Think of hills as your friend. They make you stronger.
BTW, there's a ride in Columbus, Oh called the Ox Roast ride Labor day weekend. Keep it in mind because it is an excellent one to do your first century on. It's reasonably flat.
Mileage is a mental thing. A couple of 25er's and you'll be ready for a 35 or 40, then 50. Pretty soon 50 milers will seem like nothing. After that you'll be looking at centuries and touring!