Has anybody here ever hiked or run the Grand Canyon rim-to-rim in a day, and can give me a general sense of the difficulty as compared to hiking up and down Whitney in a day?

A friend of mine is trying to convince me to hike (or run) Grand Canyon R2R with him this fall. The distance and elevation change are both similar to Whitney. I hiked Mt. Whitney a couple of years ago, so maybe R2R is of similar difficulty, but I suspect R2R may be worse.

- R2R begins with miles of steep downhill to trash your quads, and puts the hardest climbing at the very end when you're already exhausted
- it can be crazy hot down in the Grand Canyon, even in the fall
- huge temperature swings require carrying lots of clothes
- only a few places to get water
- when you finish, you're a 5.5 hour car trip from where you started

This is not a "which hike is best" contest - I'm just trying to get a sense of what to expect if I agree to do the R2R trip.