I know it is said over and over again how much one walks when in WDW, I have never really seen any details other than it's "a lot." When my wife and I spent a couple of days in WDW a few weeks ago I had on my Fitbit and tracked everything.

On Friday, we did nothing but explore the WDW property. We did no theme parks. We had breakfast in the morning at Kona, then explore the Poly, GF, and Art of Animation where we stayed. We went to the pool, then saw AKL, went to the outlets and walked around DTD. We spent time sitting in the car, while eating, at the pool, and while watching a movie. Then we went to the BW, walked to Beaches & Cream, then back to the BW for our car. How much did we walk that day?
11.89 miles, 25001 steps, with a duration of 7:15 total.

The next day we were able to get a one day park-hopper and went to the MK in the morning and were there until 2 PM. We did as many attractions as we could and we sat down to eat at Be Our Guest. Then we went back to the car and went to DHS. We stayed there until about 6. We then went back to the hotel, ate dinner and were in bed by 9 due to our flight being really early the next day. How much did we walk that day?
11.59 miles, 24279 steps, with a duration of 7:39

The amount of walking you do on a vacation like this is incredible. You don't realize it until you track it. Make sure you are ready with comfortable shoes and, if you can, acclimate yourself to it by walking or running before you go.