Ride the practice route. Based on that, you can decide if you want to go for the full century or switch to the metric route, which has way less climbing and no mountains. (Bear in mind that you would have to make that decision before you start, because the routes are not the same in the beginning.)

Whichever route you do, ride your own pace, and do not start out fast and burn yourself out early. There will be pacelines of fast riders who will be looking at the butts in front of them and not noticing any of the beautiful countryside around them. Ignore them, they don't know what they're missing.

p.s. The food at this ride is fantastic, and the Antietam Dairy ice cream at the end makes it all worthwhile. I'm doing the metric.