Do you have space on the Amtrak? I have never done a century either and I really want to do one. What has thrown me off my game is a couple of surgeries that took me off the bike for about a year and a half. I have been back on the bike since March and am feeling better than ever. Before my surguries, my longest ride with 65 miles and since then about 40 miles.
In the past couple of months, I have gotten my wife into cycling and all of my rides have been with her which has reduced my speed and distance.