It depends on what you mean by "hike the whole canyon." However you define this, I still wouldn't recommend hiking it all in one day. I did a 3-day rim-to-river-to-rim trip in early June and it was HARD. Day 1 I hiked down on the South Kaibab (7 miles, no water), Day 2 I hiked Phantom Creek on the canyon floor, and Day 3 I hiked back on the Bright Angel (10 miles, limited water). I wouldn't recommend to anyone doing all that in one day. It's HOT and dry and your body can react way differently there than it does at home. Many people get heat sickness, exhaustion or stroke when they try to overdo it in the canyon.