Loc: California (southern)
I also did rim to rim, starting from the north rim, running it in about six hours, as part of an organized group. We did the trip in early May, dodging snow drifts on the north rim and encountering 100 degrees at Phantom Ranch. What I remember is our constant intake of water. We carried at least two water bottles, one always in hand, and tanked up at every available water hole. The strategy worked. For once my urine was clear, and I felt pretty good when we topped out. I have no idea what my total intake was, but something in the four gallon range seems about right.
In hot, dry desert conditions, water is everything. Avoid direct sunlight at all costs. A wide, full brimmed hat (not a cap!) is a big help here.
Pika is probably right about your teen agers and early starts. This would be a very good time to introduce them to the early morning rhythms of the planet and how productive early mornings can be.
Have a good trip <img src="/forums/images/graemlins/grin.gif" alt="" />
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