Okeechobee is a great area for trail riding. If you don't have local contacts who own farm/swamp land where you can ride, a couple options are L-cross Extreme MX Park (http://www.l-cross.com/
) and Thundercross MX Park (http://thundercrossmxpark.com/
). Both have tracks as well as trails.
My son and I have done L-cross twice in the past couple months and will be giving Thundercross a try in a couple weeks. For us, it's a 100-mile drive and it is worth it...we've had a blast there as there is a mix of easy and challenging trail sections that we enjoy
For other places around FL, check out: http://www.outdoortrackandtrail.com/.../FL/flpage.htm