Not really any trails but the Eglin Reservation is there and its massive... might have some trouble with 2WD but hey go for it, but you do have to buy a pass to get on it or else you can get a hefty fine
Over the years I have lived in West Palm Beach, Sarasota and now Orlando and I still know of no trails. Florida seems to be lacking here. Maybe it is b/c the state is so flat and boring, not sure. I too would love to find something though.
Also, the Withlacoochee State Forest (about an hour west of Orlando) has tons of fun trails and is free (unlike HardRock). Here's an old thread that Pirate posted about a trip we took out there with RunnerUp:
Some guy posted some pics of him riding on some trails in the FWB/Destin area. I believe it was in the Gallery area. I inquired about the trails and they are on the Eglin AFB.