I did this in 1992, when I was considerably fitter, younger and thinner !
This is not for the feint-hearted. Its like climbing a mountain in reverse. At the end of the day, you have to climb back out, just when you are at your tiredest. Anybody can walk down into the Canyon for a mile or two, with the correct back-country pass but you should be aware of how difficult this trip will be. Its a very long way down to Phantom Ranch ! I am not trying to put you off, just telling you the truth.
If you do decide to do it, start as early as possible. Depending on the time of year, 04:00 wouldn't be too early. You may even have to start before it is light. Take as much water as you can carry, and then some.....your pack will be lighter as the day progresses, so struggle a bit to start with.
Usual warnings about comfortable, well-worn-in footware, and good quality socks. Also a hat and sunglasses are about as essential as water and boots.
Don't forget some sun bloc...the Rim is at 7000 feet, so the UV levels will be very high. Almost all the annual casualties that occur in the Canyon are sun-stroke and dehydration, not twisted ankles.
Also one more thing you should be aware of. The Grand Canyon isn't the amazing world attraction that it is for nothing. You may be in single file for the first mile or two, so be prepared for some jostling !