Of course, I'm sure you know what you're getting into, so I'm reluctant to offer much more advice. But I'll still offer a little. One, watch your fluids and don't drink too much water. That may seem counterintuitive, but one of the biggest health problems people encounter in the Grand Canyon is called hyponatremia. In layman's terms, this happens when people drink so much water they literally thin out their blood chemistry, resulting in too-low levels of sodium. So drink plenty water, but also eat solids and drink something like sports drinks, which contain a little sodium.
Two, brace yourself for all kinds of crap from park rangers. They hate rim-to-rim hikes, and if you encounter one on the trail who divines what you're up to, he or she will actively discourage you from doing so. To some degree their concern is justifiedthe heat and altitude of the canyon is something many people can't cope with, even very fit people. But it's also a sort of creeping paternalism that infests many of the parks. So be prepared.
Lastly, have fun! I'd love to do the rim-to-rim hike some day.
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