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.
Twenty miles is a feat for anyone IMO. It isn’t something you just decide to do. I am sure it is something that involves proper training, nutrition etc... I don’t think he will be able to walk as well. Plus, that weight will surely strain his heart and breathing not to mention his joints (knees, hip, ankles, feet). I have a feeling if he ate correctly with his current exercise routine, he would lose weight. He seemed to want to be the heaviest guy so he is gorging to keep the weight up.