I'd think long and hard about doing it all in one day in the summer, even being in good shape. The NPS advises strongly against it, and their daily incident reports this time of year are full of episodes involving dehydration, heat exhaustion, and other serious heat-related problems.
FYI, there is no water on the South Kaibab trail, and no shade either. Coming back up the Bright Angel, there is no water and little shade between the river and Indian Gardens. There are a couple of rest stops with shade and water on the 4.5 mile segment from Indian Gardens to the South Rim.
A common strategy for those who do try it is to leave very early in the morning and get as much of it done as possible before the sun gets too high. Coming back out, you're still going to need to work hard to stay hydrated.
I just got a message from a guy who got off a raft trip down there, and for a good part of the day they were registering temperatures above 120 degrees at river level.
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