Hi Teri! There is no question that when you plan a vacation at Walt Disney World you will also be planning on a lot of exercise. I have been astonished at how many miles our family treks through the parks each day.
I strongly suggest you consider renting a wheelchair for your daughter to use during your stay. The parks have
Wheelchair Rentals but they are on a first come basis. During busy times it might be harder to get one each day although I have not seen them run out. Your best bet is to call one of the authorized outside rental companies that Disney recommends and rent through them for the length of your stay. They have children sizes as well and they will deliver and pickup from your resort.
Now your daughter might be resistant to having a wheelchair as young teens are searching for their independence. I suggest you make it her project to plan on decorations for it and help her buy supplies that will allow her to make it her own. Explain to her that she does not need to be in it all the time but it is there in case she needs a rest and no free seating is available. By decorating it she can have something that no other guest will have in the parks.
If she does not want to sit in it much she can push it and that alone is a bit of a rest. Let her know she will simply be taking her own chair with her in the parks since they do not allow folding chairs or stools.
By renting from the outside vendors you can have it at your resort to use as well. When traveling to and from the parks she does not have to sit in the chair if she does not wish to on the buses. The use of the seat is up to her and you will know when to guide her to take a rest now and then. It would be money worth spending knowing she will be as comfortable as possible. The heat alone in Florida is hard for anyone and add some humidity and it becomes difficult. Stay hydrated too!