As for calories, I wouldn't just eat calorie dense foods like pizza the day before - which fat, obviously being the higher in calorie out of the macronutrients (1g of fat equals 9 kcals). Fat also takes longer to digest, especially at higher altitudes. Eat the fat and protein after the hike - not before! Mt Whitney main trail is all up hill and you will need to fill up your muscle glycogen to have enough energy to hike. Hikers that do not do this often run into trouble with low energy. Then as they reach 12k in elevation, they run into hypoxia issues (just like you described) making it even harder to get food down. You'll want to do a specific carb-loading pattern designed for your body a couple days leading to your hike. I do not know your physical condition or daily energy expenditure needs, so I'll show you what I do the days leading before Whitney or any long day peakbagging. 2 days out I increase my daily carbohydrates to 100grams. One day out, I double my daily carbohydrate intake (usually this puts me around 400g of carbs), I lower my fat intake on these days as well. Protein remains the same. The day of the hike sometimes I'll eat breakfast or I'll wait until I have hiked around 5 miles - all depends how I feel. When hiking, I mainly eat 200 kcals of carbs every hour or two. Again, this can vary depending how my body feels. Ever since I have incorporated these strategies (years ago), I have had no issues with maintaining my energy when hiking long days. You will have to experiment with your body to see how it responds. For example, I am engaged in consistent resistance training as well as cardio training. I hold more muscle mass than the average hiker. So, I have to eat more carbs (and calories) for my energy needs.
Carbs have another not so often mentioned advantage over fat. Although carbs provide less energy PER GRAM OF NUTRIENT than fat, they actually provide more energy PER GRAM OF OXYGEN needed for metabolism. In other words, when oxygen is limited, carbs give you more energy for the amount of oxygen that is available. This naturally gives carbs an advantage over fat at high altitude.
