I might be crazy, but I didn't do that much training for my first century. I did my usual hard and or long riding, but nothing more. My average is around 15.5-16.5 if it's not a route with steep climbs; my idea was to do the century at a SLOWER pace than normal! I've heard of too many people burning themselves out trying to finish. The century I did was mostly flat, but with terrible headwinds and rain/thunder.
I'd make sure you build up to a ride of at least 80 miles before the event (I didn't, but I KNEW I could do it) and find a pace that you are comfortable with. If you want to improve your speed, then intervals are the key, but I did find concentrating on a target time goal was better for me. I wanted to finish in 7 hours riding time and I did it in 6.5.
Robyn