Good for you, “fsherfy!” I’m 63, and still riding the same Colorado Front Range trails (i.e. lots of climbing) I rode almost 20 years ago, albeit a whole lot slower than before. It seems like the older I get the more it hurts, but at the same time I’m much more aware of what a wonderful gift it is to still be able to do this thing I love. I wouldn’t get too overly hung up on wheel-size and all that other stuff. Get a decent bike, get out on those great AZ trails, and be thankful you’re still able to enjoy it. Good luck, and check back in from time-to-time to let us know how it’s going.