I highly recommend you begin with a pre-cleanse diet. Eat fruits and vegetables for 3-5 days before beginning your fast to prepare both your body and your mind for the process. If you drink coffee, taper off and stop during the diet.
40 days may be overwhelmind psychologically. Do a ten day MC. At that time take stock and ask yourself if you are prepared to do another ten. Proceed in this way until you reach your goal -- even if you stop at ten days you will have succeeded!
Much of "hunger" is psychological. I have fasted to 60 days and never felt True Hunger until day 45, nothing like the stomach shrinking sensations we all feel during the first week. I was drinking the lemonade until this point, then switched to fresh fruit/veg juices which completely satisfied me and allowed me to continue to day 60 feeling wonderful!
My reason for long fasts was intensive colon cleansing, and I was supervised by my excellent naturopath. Why do you feel you need to fast for 40 days? What are your cleansing goals? In order to prepare yourself mentally, you must be very clear on this point. KNOWING why I was doing it and KNOWING I needed to do it to attain my goal kept me focused at all times.