It takes a lot of energy to backpack, and for someone who is about 160lbs and backpacking for 8 hours a day, there is a range of about 4500 to 6500 calories per day being burned depending on the speed, terrain, and pack weight from various formulas and approximations that you can do the calculations with.

Going over flat ground with a minimal pack will put you towards the lower end of that, while trying to beat the sunset going over a mountain pass at the start of your trip with a full pack will mean your burning a ton of calories and put you in the later category.

A lot of times during a trip you can’t really slow down to save energy because you’ll run out of food so its a balance of moving fast enough where you can “skip a meal” and then getting to the next town with a resupply waiting and hopefully a hot meal you can pig out on to make up for all the energy lost on the trail.

Most people pack about 3500 calories per day which is far cry from the 4500 to 6500 calories mentioned above but for most people it is too hard to actually carry enough food so its crucial to eat healthy before and after the trip and in between hikes when you can.