Researchers have conducted some very interesting studies to determine if a person burns more calories being active (or even sedentary) at higher altitudes.
This ia a really interesting article from Wired magazine. www.wired.com/wiredscience/2010/02/high-al
From what I've read, a person does burn more calories while exercising at higher altitudes. However, the catch 22 is that same person may be eating more food because the increased calorie burn causes greater cravings. increase the amount of activity you do and your body requires more energy for fuel.
How many more calories did you burn in that two hour hike ? Unless you were wearing a Body Bugg or some other calorie measuring device, there is no way to know for sure. It'll be a rough guess-timate at best.
Even if you burned say 2,000 calories, you may have been so hungry after the workout, you need to eat 2,000 calories. I know that when I do intense exercise, I'm ravenous afterwards. but do check out the article. it's interesting.