Hi, I have been there last fall. I did road cycling. Cycling is very popular there and local clubs organize regular morning rides almost every day. I would suggest you to look at
http://www.evergladesbc.com/ and join them for rides. They welcome all cyclists. It will be more interesting. Cycling as it self in the city is not really great because of huge traffic and non-stop traffic lights. Better is to go with local clubs, they know the roads.
This website is good
http://www.themiamibikescene.com/p/group-rides.html You will find there a list of clubs and organized rides. Miami is huge, it depends where are you going to stay in.
Enjoy sunny and very humid Miami. Will be fun:)