Both my husband and I have trouble walking for long periods of time. But Disney World requires a lot of walking. So we take it easy. Plan where you are going so you don't need to backtrack very often . Some backtracking is always necessary but the less, the better. For instance, we like to go to the Finding Nemo show in the Animal Kingdom. So we give ourselves time to get there well ahead of time and check out various attractions along the way. Check out times of performances. The Lion King show we can see either before or after the Nemo show. We pass it going and coming.
We always allow little breaks for "sitting a spell" and since we stay on Disney property, we almost always go back to the room for a rest or a swim in the afternoon.