I think many people, especially first-timers, don't realize how physically demanding a WDW vacation can be. You are right on the mark with the 8-12 mile experience. That's pretty typical. For folks who lead sedentary lifestyles back home, this comes as a big and uncomfortable surprise.
The two biggest things are getting yourself in some measure of physical condition before your trip. Get up and move. Go for a walk each day.
The other big thing, probably even more important than exercising, is good footwear. That does not mean flip flops. It does not mean heels, ladies. It does not mean Converse All Stars or Keds or any other worthless shoes with no padding or support. Get yourselves a good pair of walking or running shoes. Yes they may set you back $100 or more but you'll be glad you did.
As for me, I've never had a problem with the walking but I stay pretty active. I run 3-4 times a week. In fact, on our next trip in November, DW and I are doing the Give Kids the World Gingerbread Run 5K!