Well, You have your trip planned so I dont want to be a downer...but we went last year Aug 24-Sept 2.
It was soooo humid, they said it was worse than usual. I would drip as soon as I left the hotel room. We drank what seemed like a lot, but I never peed at the parks, so I know it wasnt enough.
My new husband dragged me from ride to ride- we saw everything!!.
It was not fun- in fact I hated it. I hate the summer. period, so thats not really a big help.
We bought ponchos (fun looking) as there were plenty of short term storms passing through- but very quick.
The advice folks have given you...stick with it, its so key to having a good time. Wear great shoes- bring 2 pair!!! If a pair gets wet, have a second pair for the next day. Bring LOTS of socks, carry with you in the parks, or some shops sell them, make sure your feet stay dry (lots of sweating going on down there) plus the rain, so be sure to do a foot check for dry and blister free socks.
Go take a break somewhere, I know it can be a long trip back to the room, but it could be worth it for your sanity.
Best place in Magic Kingdom to rest and cool off...Cinderella's Castle!! Its cool and dark and they dont rush you!
Be sure to swap out of rides that are int he sun, like astro orbiter, with rides in the dark, like Buzz Lightyear.
Go slow and just enjoy what you can....drink lots of water!!! Wear a hat.
It broke my heart to see kids with flip flops on and band aids all over there feet or complaining that their feet hurt...
We are going back in May to try and recreate our honeymoon- husband promises to go slow this time.

As I told him, if I dont have a good time this time...we are never going back. But I think the parks were great, the humidity killed it for me and my single pair of sneakers- they should have a shoe shop there!!!