I also did one of the hikes to the CO river in the 90s with 3 friends when we were in college. We got off to a late start so jogged down a good deal of the way (dumb). We carried a 1 gallon water jug each (not even close to enough). We had no extra clothing (dumb). Two of us (me being one) made it out that day. The other two didn't. A group of smart hikers with water filters and iodine tablets found them belly up on the path in the direct sunlight. I'm certain they would have died if the hikers didn't give them tablets, water and emergency blankets. Man we were dumb.
If you are fit, you can absolutely do this. In fact, you can go all the way to the other rim and back (rim-to-rim-to-rim). The current record is sub 6 hrs by Ultrarunner Jim Walmsley. [0]
I read all the chapters on the hiking and falling deaths before I hiked rim-to-rim. So many of them are "Family hikes down to the canyon, young child complains of heat. Sits down and dies." Or "Someone takes a shortcut, walks down an un-climbable hill. Dies of exposure."
There were a lot of completely unprepared tourists on the way up to the South Rim. They had walked down a little bit and couldn't walk back up. We weren't in great condition ourselves but we had been hiking for the past 12 hours.
Canyons can be a challenge. To maybe paraphrase some signage along the way. Down is optional. Up is not.
Going down to the river makes for a very long day. I've boated (part raft, part other) down the canyon but I've only hiked down to a spot part of the way and then back.
