I'm actually a bit surprised to read that you fasted so long without actually knowing the details about what happens to your body for an extended fast.
Your body does indeed loose most muscle for the first few days. More precisly for about 52 h for women and 72 h for men. After that, the body goes into a 'preservation mode' and the fat is converted to energy. Your brain switches from glucose and starts burning ketones.
I recommend 'Fasting and Eating for Health ..' by joel furhman for an excellent indepth read on what's actually going on when water fasting.