I did my first century last year, after riding for 6 years. We chose a relatively flat one (Tri State Seacoast ride in NH, MA, ME). Our riding time was about 6.5 hours, but we were out on the road from about 7:15 AM to 3:30? It might have been flat, but riding by the ocean on the way back, we had horrible headwinds, which were the same as hills! Plus, I had 2 flats, a result of a tear in the tire that we had to boot with a dollar bill. Oh and at about mile 85 the temperature went down about 25 degrees and a torrential rainstorm with thunder started; we had just enough time to get our rain jackets on, but not the pants. The rain only lasted about 10 minutes, but it made the last few miles miserable. We were seriously worried about hypothermia. I would say I felt fine until mile 60-70, but once I got to 80, I knew I would finish, no matter how slow I was.
I really didn't train for this. I ride on hills all of the time, which I think helped with the endurance. I consistently ride a lot of 40-55 mile rides, but last summer I didn't even do too many 50 milers. The years before, I did ride a few 60-70 mile rides. So, this isn't the best advice, but it worked for me.