Is this a mass start type event with a cut off time, or one of those where they give you a map and you go out and do your own thing? You can do 100 most likely if you're in good shape. Its just a matter of how long it takes and how much you're willing to suffer. Unlike running where your body falls apart if you don't work up to the distance over months, jumping into long distance cycling is more about fighting through fatigue. However, if the pace is being dictated to you or if you're pressured to finish by a certain time then it gets much much harder, both mentally and physically.
Oh, and don't get new pedals unless you can dedicate a lot of time to practice before hand. You'll be tired and potentially riding in a group with lots to keep you busy and distracted. More likely than not you'll forget to unclip at least a couple times.