While fatigue or generalized weakness are common reasons this may occur, there are a number of other potential etiologies.  Peripheral arterial disease, or PAD, often manifests itself as pain one experiences after walking.  The pain is caused by blockages in the arteries limiting blood flow and the delivery of oxygen to the tissues which thereby leads to cramping with exertion.  The cramping most commonly presents in the calf muscles of the legs, but can also be found in the buttocks or thighs.  Other common causes of pain in the legs while walking or at rest include venous insufficiency, lumbar spinal disease or even electrolyte imbalances.  It may be something you wish to investigate further with your provider as there are fairly simple tests used to determine the culprit.  Regardless, it is never wrong to keep walking and keep exercising!