The trip is beyond words. The size of the Canyon doesn't become evident until you start to hike in. The trails are very well maintained, but it still amazes me on how open the trails are in some places. I'm not big on heights, and some portions of the trails give you the opportunity to experience being on the edge of a 100ft+ drop, with nothing to stop you, not even a hand rail.
Rim to Rim to Rim is a lot of miles and has huge elevation changes, but I will contend that if your feet are in good shape, and you are in reasonable health, it is a trip you can make. The main failure mode of the Canyon, are problems with repetitive injuries (knees) or blisters. Most people can usually overcome being "tired".