ExMachina,
13000 ft is a tremendous amount of climbing for a first century (or 2nd, or 3rd...)
Have you considered cycling the half century - 50 mile route and hit a full century later in the season as you build your miles? The 50 mile route still offers about 6,000 feet of climbing according to the website.
Depending on where the two routes split, you may be able to make your decision during the ride depending on how strong you are feeling.
Have fun and let us know how it goes!