OP, thanks for posting on this board. My 6yo DS weighs 60 pounds and he just hates walking all over Disney-I found this thread while searching for info about Strollers. He plays soccer, basketball, and baseball so it has absolutely nothing to do with not being in shape--he justs LOVES to ride in anything---car, stroller, wagon, buggy, bike, you name it.
On our last trip, DS1 was 5, DS2 10 we rented a double stroller for the entire stay and it was TOTALLY worth the expense. Unfortunately, Disney raised the prices a ridiculous amount since then.
I say do what makes for an enjoyable family vacation--you will NOT have the only older child in a stroller.