I'd encourage shorter, more compact park days and days off from the parks...if you are always walking from the back of the park to the front for the bus for midday break and then doing the same at night...that's a LOT of not-fun walking that adds up.
She may be in great shape, but think about what kids do in the parks. They sometimes play on the water features, they sometimes run around the park features, and they have shorter steps than we do. I only ever rented a double stroller for my girls (at that age) when they got a stomach illness in the middle of the trip, and they were better, but not yet fully hydrated/caloried up to withstand 90 degree heat walks of a whole park - best $31 I ever spent at the time (and the shaded parts were also priceless). Only needed it on the 1st recovery day, but it was a lifesaver.
If you don't take days off (I always do, b/c I am a sweller, and there's really nothing much you can do to stop that except soak at night and eventually give yourself some time), I'd probably plan on getting the double every 3rd day to give her a break - especially on EPCOT day (huge park, little payoff) and maybe AK day (b/c that park retains heat like no other)...it's a good compromise, so she can do most of the trip herself, but still get a little break when she might be at a breaking point...and maybe on those 3rd days, she won't even need it...but knowing she can ask for it will probably mean she has as much fun as possible and worries not at all...