>I've read BPL's Lightweight Backpacking and Camping which references a study that estimated 6500 calories for a 225 lb person hiking 18 miles with 3000 ft elevation gain in 1 day.
I'm planning a backpacking trip at ~10 miles/day, 2500 ft elevation gain/day and figure that's roughly half the calorie expense (my total pack weight + my weight will be close to 225 lbs).
I'm assuming 3200 calories/day is a reasonable estimate, and is inline with what I've done on similar trips in the past, although I never paid attention to the numbers.
Everyone IS different in this respect, but keep in mind that just because you halve the distance you hike each day doesn't mean you can halve the calories. A lot of those calories go towards maintaining you basal metabolism which is how much you would burn doing absolutely nothing. You really should figure out how much you "usually eat" in calories, and then add in the extra calories needed to hike 10 miles and 2500 feet with a pack on. If you are 200lbs and very lean, you will need a lot more calories than someone who is 200lbs and overweight. 3200 sounds low to me, but maybe I've just got a higher metabolism??
An average male will normally burn around 2500 calories per day doing 'normal' activity. It is estimated our male ancestors may have needed closer to 6500 calories per day to hunt, keep warm, and avoid predators, and these were not fat guys!