+1 It is time in the saddle you need. Just get out and ride until you can do 30-40 miles. Then add in some climbing. Climbing changes the game, but you have to practice climbing. After a while do a 65-70 mile. Then shorter rides again. Mix it up. Learn about hydration and eating for long rides. A Century is not a race so pick a compatible riding partner and pace and you can do it. First Century, do not rush to it. Pick your time and ride. See if you can find one that does not have to much climbing and that is in the first half of the ride. This way you can cruse home on the last half.