You can prepare for a century with a Snicker's bar and a cup of tea. As others have said, anyone can go the distance but it's a matter of how good you'll feel. You'll be fine in 3 weeks with the amount of riding you're doing now, but also as others have said, get one long one in of 50+. The smiling people at the event will carry you the rest of the way. The support for the TDC events is extraordinary, so start slow and eat like a king during the ride (a king that eats bagels, cookies, turkey sandwiches and bananas). If you've never done a century before, the dreaded "bonk" will come much more quickly as a result of poor calorie budgeting than poor training.