Is she too old for a stroller?

I'm taking my little sister to Disney world. She is 7 and turning 8 in a month. She's 50 lbs. we will be walking a ton in the parks and in a lot of heat. I bought a stroller for her that goes up to 50 lbs and fits her. Is she too old for it? Should I make her walk? We should be walking at least 5 miles in 90 degree weather 

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