NAPS. In a word, naps. Even if your kids have outgrown them. The stimulas of the parks and the excitement is exhausting - and that is even above and beyond the walking. Make the trip about stuff they love. If an adult wants to ride a roller coaster or something have the other adult take the child to one of the play areas. Do not try to do everything. Let the child have fun looking at stuff and sniffing flowers and playing in the fountains. Have relaxed short easy meals and take them back for a nap. Lie down with them in a cool, dark room. Then go back at 5 when you are rested and happy. Watch all the cranky, tired, crying kids leaving the park with cranky parents and rejoice. This is a happy place. Be happy. Remember you are there to give the kids a good time, not to ride very ride n the park or to get autographs from every character while you stand in long hot lines.
I take my now just turned 4 year old DGD every year by myself - and I am an older grandmother. We both nap. We play. We watch ducks.
I couldn't agree with you more about those naps. They are important. I was fortunate to take one my last trip. The other days I ran around doing laundry while my husband & my then 7½ dd, nearly 5 yo ds & 2½ dd napped (OK my ds wouldn't nap but he did have quiet time with the lights off/tv off/curtains closed).
We get up at the crack of dawn & do the EMH park (and get there before it opens). We have lunch there & head back to our room for nap/rest time. Then we head back at dinner time with well rested kids & a dad but a tired mom but she is in Disney & figures she (well I) can live with out sleep for a few more days. My kids did fine staying up until 11 with out the cranky pants attitudes.
We do a non park day. Last trip I was lucky to have 2 non-park days to recoup.
We bring breakfast & eat it in our room. Our kids eat while dh & I take turns showering. OK, reality, I wake up at 6, take a quick shower (I take another one after rest time), get the kids clothes out & start opening the blinds at which time my dh finally wakes up. So we eat & are ready to go to the park vs us getting ready & just sitting there waiting to go & grab a bite to eat somewhere.
I am OK with the fact that I didn't ride Space Mountain in 2002. I had riden it every trip I had previously taken except in 1975 since I was not quite 4 then. But it was OK, just seeing my dd, then 15 months old, be so happy, I didn't care anymore about Space Mountain or Spash Mountain or Test Track, etc... It was all about her.
It is OK if the kids get soaked under the various sprinklers. They will dry off (but I go in the summer so the do need that cool down).
Have water bottles for the parks & some small snacks while waiting in line.
Use a stroller if your kid is still of age or even just over that OK age.
Just enjoy your self & don't over do it.