You would start by running around the block. Then you would run one mile, then two, then three, until you gradually increased your endurance to the level that you desired. So it is with fasting. You should not start with a 40 day fast. You may never want to attempt a 40 day fast, just as most joggers never attempt to run a 26 mile marathon.
Most joggers run a few miles.
That is sufficient to keep them in the physical condition that they desire. They do not wish to be in marathon condition. Fasting is a personal experience done for many different reasons. There is no reason why a healthy person could not jump straight into a long fast, if it is conducted correctly. Many of the health clinics that specialize in fasting cures would routinely put patients that had never fasted before on a 21 day water fast.