Everyman has to face challenges in his life its how you react to the challenge that makes you a man.
The mental and physical preparation that goes into running a marathon give you the foundation of skills that you need in every day life to help you succeed. Determination, discipline, ambition, stamina, desire, focus.
If you can achieve this it will impact on other areas of your life. The sense of achievement can be used as a feeling that you want to create in other areas of your life.