There is a State trail near the Van Fleet Trail. Withlacooche State Trail in North Central Florida. It is 92 miles round trip. I spend a training vacation there doing the 92 miles every day for 10 days.
The Van Fleet has fewer cross roads but is much less scenic and does not offer as many places to eat and drink.
Now to speed: I trained at 16.5 MPH because that is what the tour required which I trained for.
IMHO, a 17 MPH average as measured by a Cateye, is a pretty good speed for a 60+ guy. I can do 20 MPH average under ideal conditions. I remember some nasty headwind in Florida. It can sap all your energy. Going 100 miles/day creates issues with nutrition, hydration and saddle sores. I go along with the idea to bike more. How about 50 miles/day on average? That means some days 100 miles to make up for a non biking day? Now try to do 100 miles in under 6 hours and you got a goal to shoot for. (lots of bikers do it under 5 hours. I did it only one time)
BTW, I will go to Withlacooche again. I really liked Inverness. Charming place.