I can't imagine looking at 40 days on the Master-Cleanse as I sit here on day 6. It's not easy and requires a completely different mindset. Even Robin (from Howard Stern) did three 21 day fasts -- with breaks inbetween.
I highly recommend -- if this is your first fast -- to focus on the 10 days and then take it one day at a time :) Many (like myself) set mini goals. for me it's 10-15-21. On day 6...day 10 isn't that far off. When I reach 10, I have no doubt I'll get to 15 (as I did in my first cleanse).
Ultimately, my goal is to have a clear pink tongue -- but it's only 1/4 of the way there. I encourage you to read the book top to bottom to really reap the benefits.
Best of luck, and I look forward to reading your updates.