Having done the Rim-to rim hike - in the opposite direction you are going - I can offer a few possibly useful tidbits.
The first is that if you get on the trail early, you can be in the shade instead of the sun. I was there in june also, and by starting at the north rim at 4:30 AM, I walked in the shade almost all the way to the bottom of the canyon, which made a big difference in my comfort level.
You'll get your pack well under that 40-50 pound range witha little effort - I see that a lot of folks have offered help in this area, so i won't repaeat all of that.
I found that going downhill was not neccesarily less tiring, but I did need a lot less water on the downhill side - like maybe one-fourth as much. Going up, I drank about 2 liters per hour or more (temp at the bottom was, I believe, at least 105 that day) I think this difference was exaggerated by the difference between morning ad afternoon (down in the morning, up in the afternoon) and sun and shade (down almost all in the shade, up mostly in the sun). But I still think I need much less water on the downhill.
I did the hike in one day, and from what I recall I drank about 4 gallons of water total and was still a little dehydrated at the end. I didn't use any rehydration solutions or anything, but I did make sure all my food was salty. If I was doifn it now, i'd be making sure I got more potassium to go with the sodium.