I assume you are riding the trail along US1 in the keys. As posted above, I also have a copy of Florida's paved bike trails.... has lots of good ideas and places to explore.
We go to Maine pretty much every summer and ride the carriage roads in Acadia National Park. While they really aren't level, they were built for horse and buggies, so the grades aren't really bad, although , coming from florida, they seemed like it at first. I think there is about 40 miles of carriage roads in the park.