I am seriously going 14-30 days consuming nothing but water. 24 and 48 hour fasts are incredibly hard because of the signals your body is giving you and our psychological need to eat (trust me a lot of it is psychological). Think about it, after 24 hours without food you are definitely NOT starving even if your body tells you you are.

However, we are survivalists at our core and in the world we are biologically built to live in we don't know where or when our next meal is coming (think our hunter/gatherer ancestors). So you are programed to eat food when it is available and your body is programmed to store extra food as fat for times when food is scarce. From a survival stand point this is our body working perfectly, fat is good. The issue with this is it should be a cyclic progression. We find a good source of food, pack on some fat and then we run out of food and have to search for the next source while using up our fat stores. Unfortunately, (or rather fortunately because having food always available is a huge boon) most of us don't have that cycle in our society anymore. We eat and pack on fat, eat and pack on more fat, eat and pack on more fat while doing less and less physical activity.

However, our bodies desire to run off of glucose rather than using our fat is programmed in us at a cellular level. So without the survival need to run solely off of glucose and spare our fat stores our body is still telling us to do this which makes us reach for the next meal whether we need it or not because our body thinks "well next week we might NEED this."

When you begin a fast the first three days (48-72 hours this depends on your level of activity. Active people deplete their glycogen stores must faster than sedentary people) are normally spent out of a ketosis state. Once you enter Ketosis the ketones seem to have an appetite suppressing effect. The exact reasons for this are still being researched but the focus right now is how they effect the food-satiety center in the hypothalamus called the appestat. Much in the same way prescription appetite suppressants/weight loss like adipex-P work.

If you can make it through the first 3 days things get significantly easier. If you can make it through the first 7-10 days you'll start feeling a lot of the benefits of long term fasting. One of the BIGGEST issues with the first 3 days of the fast is the psychological issue of boredom and habit.

Sorry for the long winded response.