wiseowl, you are right in that each century is different and some are easier than others depending on the terrain and weather. 5000 ft. of elevation gain over 100 miles is very doable. As you are training for it, find some routes that have that much climbing in a shorter amount of miles.

From my experience, the biggest challenge is just the saddle time. No matter how comfortable your bike (including the saddle) is, after about 80 miles things begin to get uncomfortable. What I do to get ready for that much time in the saddle is to go on long rides on weekends - anywhere from 75 - 90 miles. After 90 miles, what's 10 more on the "big day"?