No you arent wrong, It will be hotter than Hades, but you will have a blast, we are also going this Aug. You might want to think about being there for park opening and then leaving after a early lunch (11-12) and go back to your resort for a swim and come back around 3 or so feeling refreshed...
My kids and I normally try to walk more a few weeks before our WDW trips, to get into shape and be able to walk more and believe it or not it does help. This was my DS11 idea he read it in a WDW guide book we got from the library. This way you will also know which shoes are good to where when you go, I bought a new pair of leather Keds for the trip but ended up not taking them after our walks...