Linda,
It sounds like a great trip! I'd like to suggest a few things to make your trip even more enjoyable.
Consider your general physical fitness, since several days are listed as "strenuous". My wife and I are in our late '60's, have no physical infirmaries (arthritis, bad joints) and are a little overweight, as are most Americans. When we go to Europe, we average walking 10 miles a day, which is what you'll be doing on some of your days. If you are not used to walking a lot, I suggest you walk as much as you can before you go. Buy comfortable shoes and break them in with this training.
I suggest you budget to splurge for one dinner. Put aside 100 Euro for the splurge. For lunch and the rest of your dinners, it really depends on how much you normally eat. We spent as much as $20 euro each for lunch in Florence (including wine) and as little as 3 euro each for a slice of pizza and a beer (which was cheaper than a Coke) in San Gimignano.
When in Volterra, be sure to walk out the west gate in the late afternoon. It is the village promenade and you'll see citizens strolling and visiting. The sunset on the town is spectacular - stay until after sunset.
If you aren't used to packing light, the thought of making the trip with just a carryon can be daunting. Don't worry, it's amazing how well you can do with just a few carefully coordinated outfits designed to mix and match. Read up on the packing tips. My wife and I went on a 9 day trip to London and Paris with just carryons, including business outfits for her!
A Rick Steves tour is a wonderful introduction to traveling in Europe. You'll find it is much easier than you imagine. Be flexible and take a positive attitude and a good sense of humor. I have to warn you though. Once you go to Europe, you'll want to go back again and again!
