The Okeechobee trail around the Big O lake is gravel 99% of the way. But you will not have a better scenic route. I am sure it is only 90-ish miles for a complete circuit. There are several towns to stop at around the lake. Basically you are riding a dike system built ALL the way around the lake.

The road to Key West, in my opinion, is not bike friendly. Of course it has been a few years since I was there. Maybe changes have made it more friendly. Central Florida has more than its share of L-O-N-G trails that are almost connected now. These are outlined in another thread. I am not familiar with any trails in Miami area although I know I would like to ride the US 41 Tamiami Trail from one coast to another. I have been on it in a car and the traffic is exceptionally light due to its proximity to I-75 (Alligator Alley or SW 8th St.).