Why should you try to stretch the limits of your own endurance?

Well, there are some pretty good reasons why you might want to. As with anything in life, the maxim "nothing ventured, noting gained" applies. Psychologists believe the only way for each of us to be truly happy and content in life is to know that we are striving towards and accomplishing something each and every day. The human mind needs goals, something to focus on. Our bodies too need to reach, stretch, achieve and excel. We gain energy by using energy. We conquer our limitations by literally conquering or surpassing them. Yet the investment of time you need to make in endurance sports is substantially larger than in say tennis, football or golf - but otherwise there are no other requirements needed to try to double or triple your mileage or run more marathons more frequently than you have before. Sure, someone else has probably already done it, but by reaching beyond what you think you are capable of, you stand to learn a lot about yourself.