Runners have a specific body and yours is not made for it.
This is true only if your goal is to be a world champion, Olympic medalist, or record holder. Most human beings should be able to sun a marathon in 4hr or less through proper conditioning and in their own time, but it's more a matter of dedication. This is difficult to explain sometimes, and people always seem to find a medical diagnosis or family engagement to keep them from putting the time and work in. Take this from a guys who was told not to run because he had no arches and not to cycle because of faulty knees when he was a kid. Now one of my favorite hobbies includes running and cycling for hours and the only injuries I've had are because of irresponsible decisions, not training or the sport.