First, I am not a doctor and I would suggest obtaining a doctor's permission for a fast. That said, I hope that you are in good health other than the weight.
I have used fasting to help with shrinking my appetite. I beleive that you can stretch out your stomach and when this happens you are able to eat more food. So after fasting I would find myself getting that full fealing much faster than before. That said, I never fasted for more than four days. After the four days were over, I would slowly ease back into eating.
In regards to McDonald's, I had a mother that never, ever cooked and we grew up on fast food. That said, I became a vegeatarian in college and my diet changed so drastically and I found after a few short months that I no longer wanted fast food or high fat foods. You really do crave what you eat. If you can try to eat more fruits and vegetables I promise you will start craving more fruits and vegetables. Also, try watching your carbs. Simple carbs like white flour cause for your blood sugars to spike and the end result is that not long after you eat simple carbs you are starving again.
Good Luck with fast, if you start to feel light-headed, very weak or heart palpitations please consult a doctor immediately.