Could she walk the entire time: Yes, probably.
But will it be much easier if she's in the stroller, especially given the insane crowds and heat: YES!
I was like you before our first trip this past September. My twins had just turned 4, and I figured they were WAY too old for a stroller. They are very high energy and didn't ride in a stroller much even as toddlers. So when we went to WDW we rented just one double stroller for my 2 year old to ride in (got the double "just in case" one of the twins got tired).
Anyway, our twins ended up riding in it just as much as our son. Not because they coudn't handle the walking--it is a lot of walking but not un-doable for a small child--but because it was so much easier to keep track of them in the crowds when they were in the stroller. For me, that was the main bonus of the stroller--and we went during the slow season!! I would for sure get one during the summer. It was just so much easier to throw them all in the stroller and GO when we were ready to move on.
Our next trip is this upcoming September--and we'll be renting a double again--for our kids ages 5,5,3. Now I'm just hoping all 3 will fit in the double again;-)
Kelly