Ponzy, the number of calories burned on a Whitney dayhike is generally considered to be around 6000. That's a lot of energy burned, and for most first-time Whitney dayhikers is probably the most energy they've ever expended in a day in their life. As this will be your first attempt at a hike this strenuous, bear in mind that your best bet for meals the day or two prior would consist of complex carbs in order to build a store of long-term sugar energy to draw on during the day. As has been mentioned, your appetite may decrease drastically once you're above 10K', and even foods you love may be completely unpalatable. Assume there's at least a 50-50 chance you won't be eating enough on hike day to do the trick and that you'll likely need to depend on stored energy to get you through the trek.
Those complex carbs come in the form of breads, pasta, cereals, potatoes and rice. Load up in the 36 hours or so prior. This is NOT the time to be watching your diet and eating tofu and spinach. Trust me - you will lose more weight in your hours on Whitney than you ever thought possible in a single day. And be sure to take some simple carbs along for quick, short energy bursts when you need it (basically, sugar - I keep GU chomps in my packs for this purpose). And, of course, the obligatory Summit Snickers . . .
Clinical AMS stops a number of people attempting Whitney, but simple fatigue and exhaustion is also prevalent and can be just as dangerous. Your best way to prevent this is to have your body - especially your legs - in the best shape possible, and have a store of energy in your body to draw on if/when your appetite flags. And, of course, stay hydrated - but be careful to not obsess and over-hydrate, which can flush those critical salts and electrolytes from your body. Using an electrolyte water additive can help here (I typically drop a tab of Camelbak Elixir in every other bottle of water when I fill up). If you're peeing regularly (but not overly so), and it's clear, you're good.
One other point - do you use trekking poles? If so, great - they'll be a huge help. If not, consider them. The impact on your body, legs and knees of eleven miles and 6300 feet of descent is often overlooked - it can actually be worse than the ascent for many. You're also experiencing the highest level of fatigue on the descent, and most in danger of taking a tumble, twisting an ankle or tweaking a knee. Trekking poles are great stabilizers when you're tired and going down, especially with some of the bigger steps Whitney demands of you above timberline.
Again, best of luck! Don't over-think it, but be prepared. My mantra for the bigger mountains revolves around five points: training, acclimation, fuel, pace and hydration. You hit those properly, and your chances are much better than average. Pace, BTW, to me means walking at a speed that requires fairly deep, steady breathing that you can maintain and still reasonably carry on a conversation. You want to breathe deeply (remember what causes AMS), but not because you're in oxygen debt. If your pace is too aggressive and you're panting, you're probably in trouble above 12K'.
Gary
PS - FWIW, I submitted a couple of posts
here after my first Whitney summit in 2010, following a failed attempt the year before. As many on this board can attest, I can be wordy at times so they're rather lengthy, but they flesh out detail and personal experience in my points above.
