I believe I had about 3 liters of water plus a little bit of Gatorade. I went through all of it but would bring the same amount if I did it again. Stay hydrated on the trail even if you don't feel thirsty. Make sure to have some water ready in your car for when you get back too. Contrary to what the above comment is implying, there is no guaranteed water beyond Trail Camp so don't plan on filtering the entire way up. Side note to the other user's comment: I would NEVER suggest anyone to drink unfiltered water at any trail. EVER. Most people who recommend such a thing have never seen someone go through the negative consequences of such an easily preventable illness.
Being an endurance athlete certainly prepared me physically for this hike, but it was still no substitute for activity-specific training. Find the highest peaks and the longest trails near you and spend as much time on them as possible. Wear the same layers, socks, and shoes that you will on Whitney. Use the same backpack and put just as much weight in it as you will have on Whitney. Eat the same food you will on Whitney. Do not let Whitney be the first 12+ hour / 13,000+ foot hike you do; and do not let it be the hike for you to test new equipment or a new diet on.