Welcome, and congratulations on getting such a good deal!
Let's see...you've gotten some great suggestions here.
I highly recommend gloves, not so much that you'll need them for short rides, but if you crash, they'll protect your hands from road gravel. (And I say this from experience...I've crashed several times, and as another large rider, we tend to fall hard.)
Regarding sunglasses, I've worn some schmancy ones that wrapped around my face and had interchangeable lenses and all that, but right now I'm wearing a pair of $25 ones I got from Big 5 Sporting Goods and they're more comfortable to me.
On the water bottle, get one that's easy to clean. They get pretty groady pretty quick. And when you start doing longer rides, you'll want to get some GU or other electrolyte drink powder. I like the lemon lime and the orange flavors.
I also recommend getting a Bento box for snacks and keys and such (I use this one: http://www.teamestrogen.com/prodFB_840200.html), and a behind-the-seat tool bag for your replacement tube and the tools you'll need to change a flat, and yes, take a class to learn how. It's not hard to do, but it does take doing it yourself a couple of times to feel confident about it.
The miles will come. When I started riding almost two years ago, I was on a five-mile circuit around a lake. It seemed like forever! Now I'll pop over to the lake for a quick ten miles and it's no big deal at all.
I celebrated my 45th birthday last year with a 50-mile bike ride. That felt pretty awesome. You'll get there, too. Just be patient with yourself and respect your body's signals. Don't risk an injury.
Good luck!
Roxy
Getting in touch with my inner try-athlete.