There are a number of organized rides set up by Everglades Bike Club. You might want to check out their website for rides and routes. You can also call some the local shops like Bike Tech (Bird Road) or Mack Cycle and they can help you. I live in Coral Gables and from there there are routes to Key Biscayne, Miami Beach (Venitien Causeway) and south to Black Point and Homestead. There will be a Speedway Century on Octber 30 that will be a great ride. Personally I'd suggest you stay out of Broward if you are interested in maintaining decent speed. There is some interesting people watching along A1A but far too many lights and cars.
If you need more info let me know.