There are some basic things you need to know about hiking in the Grand Canyon. First of all, for anything more than a day hike, you must make advance reservations for camping sites through the Backcountry Office. (Go to their website for details: ) Second of all, for all hikes regardless of length, bring water and food. The desert environment will dehydrate you and sap your energy before you know it. People die in the Grand Canyon every year from dehydration.
Also remember that Grand Canyon hiking is the opposite of mountain hiking. You hike downhill first and have to hike uphill when you are tired and have already committed yourself to the distance of your hike. Hiking downhill, while easier aerobically, is hard on your knees and feet. Hiking poles help your knees, and well-fitting hiking boots will prevent your toes from bumping the front of your shoes.
We met several people who hiked across the canyon in one day. I think they are nuts! The park service strongly discourages this, and they are right. No matter how early you start this hike, you will be hiking uphill during the heat of the day. It is much better to plan a reasonable day hike or an overnight hike to the river. What's the point in torturing yourself and risking death?