Timely advice from lots of people. I have my first ever century ride coming up in three weeks (Cactus Hugger in Southern Utah) and I've been told that there are lots of rolling hills and a couple of notable canyon climbs.
I've been working my way up distance-wise, with a 60-miler a couple of weeks ago, a 70-miler a week ago, and an 80-miler last Sunday. What has helped me has been to bring an iPod, bring candy/snacks, and to stop at gas stations at regular intervals to buy gatorade and/or refill the water bottles. For example, on Sunday I stopped at 30 miles, 50 miles, and 70 miles. At the 70-mile point I was starting to feel neck fatigue but the 5 minute pit-stop really refueled me mentally and the last 10 miles I was able to actually bring up my MPH average for the ride.
I'll say this... If you're not able to do any 60+ mile rides you should Keep a really good pulse on things like finger/hand numbness, neck fatigue, butt soreness, knee aches, foot hot-spots, etc. during your 40-50 mile rides. If you feel any of those things during 50 miles you'll REALLY feel them by the time you reach 100.
I'm coming at this from a running culture and I must say that the typical century ride seems way more laid-back than a marathon. Nobody would stop in the middle of a marathon to have lunch but it happens all the time during century rides--and its a GOOD THING, imo. Have fun, meet folks along the way, stop frequently to refuel physically and mentally.
--Steve