I rode my first century in 2004. Before that, I rode many 50 to 75 mile journeys over the years. But, putting in that extra 25 miles really was a bear the first time....
Note: I was riding a big heavy schwinn with platform pedals while wearing a cotton t-shirt and cut-off jeans over cotton underwear. Um...obviously not your best century-riding bike nor was I wearing the proper apparel. BUt, I made it----in about 12 hours.
THese days, I'll knock down a C in about 6 hrs without any stress. I also now ride an aluminum racing bike and wear Lycos for said centuric jaunts. My advice for training is to ride, ride ride.