Jon: 42 miles if you go South Kaibab - North Kaibab - South Kaibab. Bright Angel - North Kaibab - Bright Angel is a shallower slope but longer.
But it ain't the miles, it's the vertical feet. A mile down, a mile up, a mile down and that final, grueling mile up. I've done it when I was half my age and that last vertical mile up was a b***h.
If you've done 10,000 feet (up+down) in a day, you can probably extrapolate what R2R2R is like. If you haven't, you maybe ought to do a 10K vertical day first.
That may sound arrogant, but truly, I mean it compassionately.
But if you're 23 and haven't done any training yet? WTH, go for it. Worked for me. I couldn't walk for 5 days afterwards, but I'm still glad I did it.
But this time? I'm training more and maybe hiking less far.