Plan your day (like the above poster, avoid criss crossing back and forth).
Every ride (or just about) you get to sit.
Take that mid day break to recoup and nap. My kids who were not nappers, napped at ages 5, 7 and even 12! OK and my 17 yo napped this summer too!
It is tough to not have that stroller to store your bag so take as little as possible. Dh and I each have a bag and each of my kids has a bag and they have to carry their own water (and phones). When our youngest was 7, she had a hard time with the bag so dh just carried her water.
Try to take some walks before hand, even if they aren't the long disney walks. I can tell you over the summer we hit 20-25000 steps on our park days with 2 parks and a mid day break. One day myself and my 2 older kids did 3 parks so we made it to 30000 but we had to walk from the BC to the studios for early entry for TSL. The pool did wonders for my achy feet on our mid day breaks-I didn't feel the need to nap since we had every other day of parks alternating with every other day of non park/pool days.