A question that a lot of runners ask when they get tired is if the legs or the body limits the distance one can do. The answer, of course, is both; it depends on the circumstances, and different people will have different answers to the question.
Since my blood clots in January, my legs have been the limiting factor. I've been combining running and walking, and I usually run until my legs start to show the stress, and then I walk for a break. Today, however, my body was the limiting factor. My legs felt great while I ran, and I stopped to walk when my body told me it needed some rest. My legs didn't feel tired at all during the whole distance. This is a really good sign!