I got my first road bike in March. I've put just over 2000 miles on it since then. Yesterday was my longest and hardest ride. 87.35 miles, 7295 ft elevation gain, average speed of 14.2 mph. (
Garmin Connect link) I probably could have made it another 13 miles, but didn't see the point. My mission was complete. Now that I've made it to the top (well, almost), I'm going to start increasing my speed on the way up.
If you want to do a century, then simply keep increasing the distance until you reach 100.