Hi all, total newbie here!

[Nandy]

That sounds like an awesome trip! You'll be fine, just take it easy and fit riding into your daily routine. Your body will adapt pretty quick- once you get some base muscles and find a proper fit, you can ride endlessly.

[pinkychique]

The best thing you can do is ride!

A good bike, with a good fit, and a great saddle will get you what you need. Quality shorts (with differing chamois between them) are important for long trips to change the pressure points on your bum.

Good for you signing up!

[azfiddle]

Welcome and good luck.
I started cycling again at age 53, about your size. Just ride a lot and keep adding more mileage, and hills if you're going to be in hilly country. I don't know what the weather conditions on your ride will be like- but I would imagine more hot and humid, so you'll want to get some help on how to ride in those conditions. Lots of water, I'm sure.

Good luck and keep us posted as you get ready for the big adventure!