There are a lot of parallels when I think about training for a marathon (or any distance) and the journey of life. You start out with a dream or vision of what you want to achieve (be that just finishing, enjoying the event or a specific time). You set a course of action to get you there that is not just physical but emotional, strategic and that requires continual course correction along the way. And then you move forward, sideways, sometimes backwards, gaining experience and growing along the way.
Everybody knows on some level how important it is to keep active. The primary reason being that being physically fit reduces your chance of developing a raft of illnesses. It's also credited with helping maintain a healthy weight. Which is somewhat related given obesity presents similar health challenges.