Mid Florida, say North West of Orlando, has the most access to riding. Not too far from Ocala national forest which has riding. Not too far from Tampa, actually just North in Brooksville, where there is Croom (A state park just for dirt riding, bikes and ATV, no trucks or jeeps) with 2800 acres and camping. Reasonable distance from Gainsville, big amature national MX race there every Thanksgiving. Several MX tracks near Orlando.
I moved down 5 years ago and live in North East Florida. We have two tracks south of Saint Augustine, Ocala is a reasonable distance, and North Florida MX track (Hwy 1 right by the Georgia border) has a clay track, a sand track and some trails. That is where we end up most of the time as my son always wants to be on a track and I like to ride trails. It is the only place I know where we can do both.