I have done a 40 day fast before to completition, it was not very bad at all (of course I was a bit overweight which makes it easier)... the first 3 days you're hungry.. days 3-10 you kind of miss food, and then after that its just a mental race to the end.. I did get some cramps and heart palpitations late in the fast due to potassium deficiency but other than that I had 0 issues.. wasn't hungry at all on day 40 and was kind of sad I broke the fast.. felt like I could have done 20-30 more without much issue.
The common misconception is the person is starving, but this isn't true.. after 48 hours or so we enter ketosis and the body eats its own fat to survive.. it also selectively eats bad tissue for energy since it has to decide on what stays and what goes meticulously... I found deep in my fast my body was eating scar tissue, and all kinds of weird things.... I highly recommend everyone if able to, to attempt this atleast once... you'll lose a lot of weight and its very enlightening.