Just because I'm bored tonight, I went to geodistance.com to map out our routes in 2 of the parks based on my notes from our trip a few weeks ago (I keep track of what rides we do and what order we do them in along with when I left the group to be the FP runner). On our Epcot day, we were at the park by 8:25 and we got back to our hotel (AoA) at 6:15. I walked 6.5 miles that day including all of my FP runs, walking to the bus stops, walking back to our room, back to Landscape of Flavors to eat, etc.). We covered all of Epcot that day including walking all the way around World Showcase.
I also mapped out my day at MK because I did a lot of criss-crossing the park for FP's that day. We were at the park for rope drop, left the park around 3:00, returned to the park around 6:45, and left at midnight. Including all of my running back and forth for FP's and walking at the resort, I walked 7.25 miles. I'm sure there is a slight margin of error with my distances since I didn't keep track of exactly when and where I went to the bathroom but I don't think that would add up to more than a half a mile since we usually just stop at bathrooms that are on our route anyway. I also didn't record any walking around in a restaurant or gift shops -- again, those distances would be minimal.
I'm sure there are people who walk over 10 miles a day at Disney, I'm just glad I'm not one of them. I know I can walk at a 3.5mph pace for about an hour. After that, it drops steadily. And that's with no obstacles or people in my way (walking on a walking path at a park). It would probably take me 7+ hours of walking time just to cover 15 miles at Disney and then I'd still need time for riding the attractions and watching shows...not enough hours in the day for me to do it. But kudos to those of you who can cover those miles day after day on vacation. My feet, knees, hips, and back would never let me do it (and I work out with a personal trainer twice a week and walk 3 miles a day two or three other times each week.)