I went on a backpacking trip in New Mexico when I was 17. Half my body weight on my back, holding everything from my tent to the food and water I needed to make last for the duration of the trip. Nearly 10 miles every day for twelve days in an elevated, dry heat. Wake up, hike, get to camp, tie up everything that could even remotely have a scent in a bear bag, go to sleep, do it over again. You start to forget what normal life is like after day three.