I'm planning on doing the Montauk metric century too, I think its a good way to start the season. It is very flat! Its also a beautiful ride. For hills, I mostly wind up doing repeats on the large hill in the north part of central park. There is a path you can take which allows you to simply do the hill over and over without cycling around the rest of the park. Its a bit less crowded in that section of the park and good practice!