One payoff from exercise is that you will have a better sense of what you can accomplish. In an emergency, you will be much more confident if, say, faced with the necessity of hiking ten miles, you know that you regularly run six or so, twice a week.

A rule of thumb from my running days is that your limit is about two and one half times your normal workout distance. Thus if you run eight miles regularly, you can complete a marathon of twenty-six miles, 385 yards. You won't be moving fast, and it won't be fun, but you will complete it.