I just finished my first water (and tea) only 36-hour fast.
Previously, my longest fast had been around 22 hours.
The hardest part was sleeping the second night in a row on an empty stomach -- I started dreaming about food and dreamt that I accidentally broke my fast! (sometimes, while fasting my kids bring a snack or piece of fruit to me and put it in my mouth, and if I am working, I might accidentally just eat it).
The most challenging part is to eat in moderation after that, but we managed to eat as much as I usually eat after a 16-hour fast.
I just ate fruits, veggies, cheese, a small loaf of bread at night, and I might go at it again for 36 hours. My goal is to build it to 72 hours and maybe even go beyond.
(BTW props to @POTUS for all the motivation).