Also park services advice against hiking down and up in one day. If you are very well trained and have done very tough hiking before, you might be able to do it. I went down the South Kaibab, camped at the Bright Angel camp site and came up Bright Angel trail next day. But the time we got to the camp site, we could hardly move. Exhaused and knees hurt really bad. Even Motrin did not help much. :)
In all, August is too hot for such a hike. Just my opinion. :)