It got my ankles the hardest going down.
Make sure you have some carbs for the way out.
Good advice. I was 33 and in good shape from a summer of fieldwork when I hiked the Bright Angel trail down to the river and back. I took several quarts of water, but no food, and I hadn't eaten before starting. Ran out of gas about halfway back up, and had to stop to rest every few hundred yards for the last couple of miles. Good learning experience.