---A warning about endurance hikes---
I don't recommend them to anyone. I've been doing these things for several years and I am very aware of what's going on in my body when I do them.
Back in the early 2000s, I performed my first severe fast - three days in a tent on a rise outside of Selma, Alabama with no food or sleep and minimum water. I was guided by a Native American that knew a lot more about how to do a severe fast than I did. For instance, he taught me that I needed to protect my kidneys by drinking water acidified with lemon.
I started the endurance hikes short and leisurely and built up to this last one, paying very close attention to what my body was doing. In addition, I have had a lot of training in human physiology. I know what dehydration looks like (the darker the urine is, the more dehydrated you are, and if you pinch the skin on the back of your arm and it doesn't snap back, you need water), and I know what fatigue residues smell like to me. I can tell how worn down a person is by their scent.
I was dismayed Friday to find that the tube from my water bladder had frozen solid. It's the first time it's done that and I've hiked in subzero weather. This time, I hiked from 5:00 to after noon without drinkable water. I monitored my hydration very closely.
Yesterday, the day after the hike, I sweated ammonia. That happens in severe fatigue. I was burning the protein from my body. I was using muscle mass for fuel. That's not particularly good for you. I loaded up on protein yesterday and today and rested. even today, my urine and sweat smells like sour milk - fatigue residues.
I have heart problems - some plugged arteries and a weak valve. I know what they feel like when they're being pushed too far and I know how to breathe and pace myself to take stress off my heart.
The big thing is, if you decide to start doing endurance hikes, talk to a doctor or exercise specialist to find out how you need to approach it and how you need to limit yourself, or whether you even should do it at all.
I push myself to my known limits - never beyond. If you push yourself beyond you could easily kill yourself, so be wise.