Easier not just not eating
I remember going without food for a couple of weeks (I drank juice and tea) and still being able to cycle 7 miles two and from university and attend classes without any problems. Once the initial hunger is over it was pretty easy and I think most of us could do this for about a month without any problems. The body just uses up all the energy stored as fat.
Going on a crash diet of reduced calories confuses the body as it doesn't know what to do. This often leads to the body trying to store the energy from the food consumed and leads to weight loss. The body needs some time to adjust to the new regime.