Hi there countrygal
I agree with you completely that a long fast is the best gift you can give to your body in the quest to vanquish these ailments.
A long water-only fast healed my lungs of an inability to take a deep breath among many other aches, pains and other various ailments. 40 days would be quite difficult and even unachievable for most people, so i would encourage you to listen to your body and take it day by day, especially after about 30 days.
The physical hunger should be mostly non-existent after the first 2 or 3 days which makes it very nice, but the desire to eat can be quite persistent throughout and additionally you may encounter some immense weakness and even nausea from detoxing. The nausea in particular makes it a lot more grueling in the days where this is present (if at all).
I'm not saying you can't reach 40 days, but if you are able to you are a rare soul. Though i wouldn't have minded reaching 40 days myself, i made it as far as 36.5 and strongly believed my body was telling me it was time to stop and that it may have become dangerous to continue further. Ideally you will be able to end the fast feeling good as i was able to, enduring nausea my last few days and taking the opportunity to break as soon as this had lifted.