I don't know if you can ever be totally prepared for the amount of walking you will do spending a week in Disney World. My family was curious a couple years ago and wanted to know how much walking we did on our trip. So my daughters brought along a pedometer and tracked how many miles we walked. At the end of the first day we all guessed, but nobody was anywhere close to the actual 14 miles we had walked that day. In the days that followed we walked in the range of 11 to 14 miles a day.
One thing we've learned over the years is that it's necessary to get your Disney legs prepared for all that walking. Because even if you walk at a slow pace, you're walking so much more than you would at home.
So if you have a trip coming up, it's a good idea to start walking and get your legs and feet ready. And don't forget to check first with your doctor if you have any medical issues!
Since several trips we planned previously were surprise trips for our daughters, we'd use our walking time to plot out the details of how we'd surprise them.
Good supportive footwear is another necessity for all the walking in Disney. I've taken many summer trips and hate to wear sneakers but good supportive sandals can be found at some shoe stores and specialty footwear stores. I've found that having a few pair of good supportive sandals or sneakers to alternate through the week works best for me. I do take a pair of flip flops for trips to the pool and water parks but I don't wear shoes without a back to the parks.
Blisters are almost inevitable so you may want to pick up Band Aid Blister Block or Body Glide. Both look like a stick of deoderand and can be rubbed on your feet to prevent you from getting blisters. If you do get a blister, Band Aid makes a nice blister pad that works real well.