I'm always amazed at how many steps/miles some people are able walk day after day at Disney. My feet, knees, hips, and back would never survive 7 straight days of 10+ miles a day. I keep track of my miles at Disney and my longest day ever was about 10.5 miles at Epcot -- and that included a little crisscrossing of the park. Most days we average closer to 7-8 miles. We definitely try to plan our days so that we're not constantly running from one side of the park to the other but rather make one big loop around each park, so that might help in keeping our miles down even though we spend long days in the parks.
My son and his wife used two different tracking methods on their last trip (because he's a technology geek/numbers guy) -- one GPS oriented and the other one was (calibrated) step oriented. Even though he and his wife were together the entire time and walked the exact same places, the step calculated distance was about 20-25% longer than the GPS oriented tracker. He also came back to his room some nights and retraced their steps using an online distance tracker and that was usually about the same as the GPS tracker his wife used. His theory is that the step tracker was calibrated to his "normal" stride, but a large part of the walking they did at Disney was not normal strides. There were a lot of steps taken while standing in a queue or just browsing through a gift shop that were about 1/2 the size of a normal stride. So while he felt the number of steps was probably accurate, he didn't feel like the distance calculated by those steps was accurate for that situation since many of his steps were much smaller than his average stride (though he said it's much more accurate for his every-day walking and running). Regardless of which method is more accurate,
Disney vacations will always include *a lot* of walking.