I'm 65, and (at least for planning purposes) also use 10-12 miles a day as a reasonable estimate. I've never done more than 15, and I have been with beginner groups that stopped at 8 - which was a short day of about 6 hours of hiking with some longer stops for teaching presentations.

But, for my own trips, 10-12 seems to be the sweet spot. After all, it's not really about the mileage.