We take extra vitamin D and C. Vitamin D plays an important part in your immune system - and is as important as vitamin C. I get proper dosages for my kids from our pediatrician (usually by age/weight), and she helps me decide what my husband and I should take. She's a fellow Disney lover, so she knows how important it is to stay healthy while there!
We also try to fit in family walks, etc. to help build stamina, mostly for the kids. (They're 4 and almost 8.) I also take them all kinds of places beforehand and we "practice" walking together in different formations - side-by-side, single file, etc. It's one thing to navigate through a store together, but something completely different to manage to get out of MK after fireworks!
We also practice what to do if they get separated from my husband or me. They have ID bracelets and I remind them how to find a CM to help them get back to us. We also go over other safety measures so they're fresh in their minds.
Also, things like wearing shoes that are well broken in. And I make sure the kids aren't just about to grow out of those shoes! Remember that their feet will swell with walking & heat too, so if they're close to growing out of them, I'll go ahead and buy new shoes for them in enough time to get them broken in.
I agree with another poster about soaking your feet in cold water in the evenings. I didn't go as far as putting ice in the tub, but putting my feet in cold water for 5-10 minutes made me (and my feet) feel SO much better at the end of a long, hot day in the parks.