If you're planning a multiday trip and will be exiting the canyon on a different trail from which you entered, plan your trip so you're hiking down the maintained trails and hiking up the unmaintained ones, if possible. Your pack is heaviest at the beginning of a hike and going down a hill is just plain harder on the body. The maintained trails on the south rim (the Bright Angel and the Kaibab) are wide, simple to follow and don't require climbing over rock slides, around tree branches etc. It's an all around easier way to get into the canyon. On the Bright Angel, there is water at Indian Gardens, so the only water you need to carry down the trail is what you would need in that 4.5 miles.
Know how fast you walk. Always have an approximate idea of how long it's going to take you to get to your next destination. Knowing that on level ground carrying a 60 pound pack you can cover 2mph will give you the confidence necessary to head out across the plateau and know when you'll be at your campsite or the next water.
Bring along a good topo map and compass and know how to use them. Plot your itinerary and destination for each day and try to stick to them.
Build in some free time. Don't plan on hiking 8 hours a day. Plan on 3 or 4. Have reasonable expectations of your abilities. Slow down and enjoy the canyon. Get into the rhythm of the rocks. Plan layover days so you can explore the side canyons, take a hike down to the river, study the geology, flora or fauna around you, read, contemplate, slow down.