Ran R2R2R on Tuesday. Great day. Took South Kaibab both ways. Here's the data:
Trailhead to River - 1:00
River to Cottonwood CG - 2:20
Cotton Wood CG to North Rim Kiosk - 3:58
North Rim Kiosk to Cottonwood CG - 5:00
Cottonwood CG to River - 6:03
River to Trailhead - 8:11
Not exactly Mackey or Krupicka but good enough for me. I'll post a detailed report this weekend.