Individual capabilities vary widely, but when I read "20km", "24km", "40km" it makes me think that there is a mental template of what one should be able to do, not what one can do. If you have that template/expectation in mind and you have not been training, you are setting yourself up for injury and an early end to your walk. If you really are out of shape when you start (and are not physically "young"), set your expectations for a much lower number. Ten to twelve kilometers might be a good goal, particularly if the terrain is challenging. If you have a plane to catch or limited time, lengthen the day with a bus or taxi, not by pressing your physical limits. If you have not been training, you really do not know your limits, and finding them by pushing past them is simply not a good idea. Injuries tend to cluster at the end of the day, when you are tired and a bit off balance. That is when you trip, fall, or twist and ankle. That is when you push a little too hard and tear a muscle or strain a tendon. That is when the repetitive stress causes tendonitis or shin splints. That is when you change your body mechanics to accommodate a pain, and start stressing entirely new muscles and joints.
Take the heroic accomplishments of the braggards with a grain of salt. We all could do more when we were younger, and lots of times we exaggerate or minimize what we really did and what the end result was. A pilgrim may be committing him/herself to a half marathon a day for a month. No one would start an exercise program with the "day-30-plan" for the first day. I recommend that you set a realistic Day 1 Plan for the first day, and build slowly. It is not hard to find stories in the Forum of under-planning and over-execution. Press yourself physically only when you are certain of the outcome. You can do more than you think you can do, but it is also possible to think you can do more than you can.
