In my 20s I did several longer fasts - water only. 7, 10 and 10 days, if I remember correctly.
Thought they were a breeze! Couldn't understand why everyone around me thought I was eccentric and killing myself, and so on.
It was a lot less effort than eating, and I felt great. lol.
Now, in my 50s, my body is MUCH less comfortable with extended fasting.
I regularly do 16 hrs fasting, 8 hours not, and the odd 24, but fasting the second night always leads to my body playing silly *******.
I have decided to listen to it.
I believe it is something to do with insulin resistance levels, and my body being extremely unwilling to access my copious fat stores. But that is just a theory.