This happens for several reasons such as incorrect training practices and starting out too fast. Long slow distance training helps the body make the physiologic changes necessary to utilize the fat burning system for energy. Rest or recovery is an important part of training. A part of increasing endurance is to arrange your training schedule to allow you to recover from and adapt to these 20 mile walks. A good training schedule alternates hard days with easy days and hard weeks with easy weeks to allow recovery from and adaptation to the longer walks.
Many athletes seem to feel if a little is good, more is better.
More may not be better, if the body is tired, sick or injured or the psyche is burned out from too much. The marathon training schules I suggest alternate shorter and longer days during the week and alternate 16 and 20 mile long walks. The third key to a successful marathon is some work at the walking pace you want to maintain for the 26 miles. Walking is a motor skill, especially as the pace increases. Motor skills need practice to develop and maintain.