Went on a backpacking trip with my best friend when I was 17. Section J of the PCT, 75 hard miles in the cascades. I wore nylon shorts, cotton socks and boxers, and an old pair of running shoes. I was fit, but the longest hike Id done previous to this was maybe 6 miles. My buddies dad knew an ultrarunner who'd done it in 2 days, so we viewed that as the time to match.
We actually made it almost halfway on the first day, 34 miles, but at a terrible cost. My inner thighs were mincemeat from chafe, terrible blisters, sun burned, and covered in bug bites. We slept under a rainfly, propped up by trekking poles, near a small lake. The mosquitoes feasted. By noon on day 2, after drinking almost no water, I was dehydrated to the point of hallucinating, and had gnarly flu like symptoms. Shivering, chills, aches, dizzy. The chafe between my legs had turned to open, pustulating, bleeding sores by this point, and I feared it was sepsis. My joints had also swollen, and were very tender. Each step I took my body was in intense agony. To be in that sort of pain and exhaustion over such a long duration, knowing it could be days before we made it out, was a type of suffering that I cant adequately put into words. Eventually I collapsed, around mile 50, luckily not injuring myself in the process, at which point my friend forced me to drink 2L of water. I fell asleep against a tree, and when he woke me up an hour later I felt strong enough to keep going. We made it out the afternoon of day 3.
It took me a few years to go out again, but Ive since done a lot of backpacking, including long stuff. In hindsight, it came down to poor clothing choices, and lacking the experience to know that I need to drink water even if I don't feel thirsty. Luckily my friend didnt make the same errors, or else we would have probably needed to be rescued. Whenever people ask me for advice about backpacking, I always implore them to drop the extra coin on nice socks and underwear. It matters.