Welcome to the ACA Forums!
First, I love WeatherSpark but remember that these are averages. I actually did a ride very similar to yours. I rode from Miami, down to Key West, ferry over to Fort Myers, up the central part of the state, then it the coast east of Apalachicola and then hugged the coast to Freeport before heading toward Austin. The only memorable winds were the section from Navarre Beach to Pensacola (which I have ridden again since the other tour) and both times I have ridden this very scenic section I had a strong tailwind (E>W). If you want routes or info, you can private message me and I will be glad to help.
Additionally, elsewhere winds were not that strong. As far as the distance, you already have enough experience. Just imagine stringing together a few cross-Texas rides and you got your ride. Your age really is not that unusual. I regularly tour with people in their 70s and low 80s. They are a bit slower and we do shorter daily distances but still.
I would suggest you check out the journals over at CycleBlaze and CrazyGuyonaBike and you should get a good idea as to what it is like.
Finally, you might consider reversing the route. That way you would have a better chance of favorable winds but as mentioned any wind direction is possible. Plus you don't have to have a set time to get home by in case of weather, mechanical, or health delays.
One thing I would highly suggest is that if you hug the coast like I did and go through Tyndall Air Force Base (or any base for that matter), do not under any circumstances exit the base around 4:30pm on a Friday night. I have ridden for 45+ years and that is easily one of my top 3 scariest times crossing that bridge when thousands of 20 something males are raring to get off work and head into town on a Friday night. Other times of the weeks are not so bad but definitely avoid Friday late afternoons.
Tailwinds, John