First, Florida State Parks accept reservations 11 months in advance. That means November, 2019 is pretty much already full at the most popular parks, so you need to get online and make reservations FAST!
Second, Florida State Parks allow you to leave early and get a refund for days not used if they are at the end of your reservation. So, if I had a week remaining on my reservation, and I decided to leave tomorrow, I could check out in the morning and get a refund on the remaining days. You might want to verify this, but it has been true in the past.
Third, there are a handful of COE campgrounds in Florida along the Okeechobee Waterway. Great places with locks and dams along the way, but very popular at $15 per night senior rate. And another great place is Midway Campground, part of the Big Cypress National Preserve, also at $15 per night for seniors with a federal card. And don't skip Shark Valley for a great bike ride.