I'd like to jump in on what Steve and Rod had to say about food to bring on the day hike to the summit.
I'm making my first attempt (going solo) to day hike the main trail in late August. Staying at Horseshoe Meadows the weekend before (2 days, 2 nights). Then it's WP camp for 3 days. I'll get in days hikes to get used to the climate while I'm there.
What is recommended to eat the day before I go up? Then for the day of the hike itself, I hear about bringing foods and snacks that you like. But carbs and calories are mentioned as a must too. Any suggestions on what types of things I should bring?

Thanks for the tips on filling up my water along the way. That will help keep my pack weight down.
Your advice is greatly appreciated.