Hey look at Alafia River State Park. I am going there on Sunday for the first time after riding at Carter Park mostly, which is near Lakeland. If you look at Swampclub.org they might know of something closer to Naples. Good Luck.
For anything close to hills(florida mountains)... You need to come north. Alafia is a killer ride and plenty of people out there to ride with. CArter is a blast. but is only like 6 or 7 miles. I ride the loop then ride it backwards, to get a little more out of it.
I rode Alafia on Sunday and it was awesome. Don't be overwhelmed by the amount of people in the parking lot. It seems like it is packed but once you start riding you barely ever have any problem with traffic. The trails are very well maintained and some of the best I have seen since I moved home from Vancouver, B.C. I can't wait to go back.