Sitting around home in 90-degrees is way different than backpacking in 90-degrees. The air temperature is augmented by the radiated heat off the rock walls in the Grand Canyon. Temperature in the shade can be 90-degrees and what hits your body is 100+. Be careful.

When we did the Grand Canyon, lots of newbies we met at Phantom Ranch were shocked the next day by their incredibly sore "downhill" muscles. Somewhat like how you feel after the first day of skiing if you did not train for it. Take it easier than you would think going down. A 5000 foot descent can really play havoc on knees and muscles. Even though you can go down and up in one day each way, we split the up into two days, spending excess time walking along the Tonto Trail - really scenic and a nice break to walk without the full pack. Were I to do it again I would also take two days going down. You would see more and you could confine backpacking to the wee morning hours when cooler.