Jt33, focus on vertical gain as much as you possibly can. Conditioning, strength and mileage are all great benefits, but a fourteener like Whitney will stress your body the most through the elevation gain (and loss). John's advice is spot-on. The only thing I would add is if you stick with treadmill workouts, kick the incline up to 15. On Whitney, of the 11 ascending miles over 10 of them will be sustained inclines. An under-appreciated fact is once you summit (we're being positive here), and your legs are screaming, you then have 11 miles of sustained downhill to deal with. It's hard to prepare your body for that unless you're actually doing it in training.
You're in SoCal - take advantage of it. Hit Baldy, Sanny J, Gorgonio, whatever, but get high and see how your body reacts to 10-11K' after a long climb. Do this early rather than later - that way you have time to adjust your regimen if you experience difficulties. Maintain a pace that keeps your breathing deep and regular, but not overly so. If you find yourself panting or gasping above 12K' you're inviting the AMS gorilla to jump on your back, and that's usually game-over.
You didn't mention your hiking background. If you don't have experience on big mountains like Whitney, there's more advice that can be offered. If you can clarify that it will likely assist anyone responding to your questions.
Best of luck.
