Check out the articles on lightening up your pack, selecting gear, sample gear lists on the home page of this site.

Trying to get in shape with a pack heavier than what you're going to carry is most likely to cause stress injuries. I suggest hiking with a very light pack, up and down lots of hills. If hills are not available, try stairs. Add weight and mileage very gradually over time until you're fairly close to what your actual pack weight and mileage will be.

Remember that a gallon of water, which is recommended for Grand Canyon hikes per person per day, weighs 8 lbs.--which is why you want the rest of your gear as light as possible!

I highly recommend several overnight trips beforehand to learn skills. You don't want to be setting up your shelter for the first time by flashlight after a long day in the GC!