If this man can jog six miles every Saturday and still walk three miles every day, he must be in some sort of decent shape. Of course, I am thinking if you are exercising that much, why do you still weight 400 pounds? Interesting.
Calories-In > Calories-Out
I know from some marathon and triathlon experience, if you haven't trained properly, things get exponentially more difficult as the race goes on. He may be able to jog six miles, but by the time he gets to mile 20 I bet he won't be able to walk.