When I first got interested in bike touring my goal was to ride down to the Keys .The trip reports and pictures from the Escape ride was a large part of the motivation. After a trip around the southern part of the state with some other members of bikeforums I did the trip down to the Keys and back twice,though this was over ten years ago now.If you're looking to get through South Miami take the Old Cutler Bay route.South of Homestead, Card Sound Rd is much more scenic with less traffic than the straight shot down US1.On the Keys, if you use the bike trails be aware they cross side roads frequently(particularly around Key Largo) where the traffic will not necessarily see you so be prepared to stop and use caution at these intersections.The main road also has a bike lane pretty much the whole way along with plenty of traffic.What I didn't like was at all those same intersections the bike lane is routed on the inside of the turning lane so to avoid being cut off by cars I would continue straight ahead on the left side of the turning lane letting any turning cars use the bike lane(and rest of the road).
December is probably the best time to do that ride as November is still technically the wet/hurricane season and during the wet season the Keys can be hot,humid and buggy.Trust me, as I did one of the rides during July and was chased by mosquitoes at dawn along Card Sound Rd, eaten up by noseeums at Long Key SP(the ranger assured me there were no mosquitos),and had a water spout pass over my tent camped on an old bridge east of Key West(I saw it forming first out over water).Not to mention while it was 100+ and humid during the day, I actually did my first century in those conditions.