On October 14, 2007 I hiked the Grand Canyon Rim to Rim - that's South Rim to North Rim. That link will take you to some photos of the hike. It's a 21 mile hike with an elevation start of 7,000 feet which drops to 2,400 feet at the Colorado River and then a climb back up to 8,000 feet on the North Rim.
It took exactly 10 hours to complete. It would have taken me more like 12 hours to finish if I hadn't been walking with these 2 extremely fit girls who kept me moving a brisk pace. They also kept a close eye on my fluid intake and made me drink frequently which is probably the only reason that I did not end up dehydrated.
Amazingly I did not feel stiff the following day. In fact my previous 2.5 hour hikes over Camelback in Phoenix left me in far worse shape the following day than this 10 hour hike did. I think that it was because of all the electrolytes and Gu Gel shots that I was taking during and after the hike.
The hike started at 4:50am and we arrived at 14:40pm. It was described to me as 2 hikes; the first was the 7 hours to Cottonwood campground and the second was the 3 hours out on the canyon. The two to three hours is grueling. People mention that it's because you have already been going for so long and it's non-stop uphill out the canyon - which is true. There are two other factors that were never mentioned which I think play an equally important role in your struggle out the canon: The altitude, there's way less oxygen up there, and the fact that you've been up since 3:45am. Irrespective of what you've done during the day you're doing to be tired by that time in the afternoon if you got up at 3:45am.
I elected not to hike back this year but intend to do the hike in both directions next year. Valerie and Dede (my amazing hiking buddies) hiked back the next day and most of the rest of the 12 person team that hiked across hiked back the day after that.