"Jack Kemp, Dole's running mate in 1996, was unfit because of a knee injury, though he heroically continued as a National Football League quarterback for another eight years; Pat Buchanan had arthritis in his knees, though he soon became an avid jogger. "
This is grossly misleading.
NFL quarterbacks can get away with having bad knees because they do so little running. Look at Joe Namath. Look at John Elway (who is/was missing a tendon!). Lean over, take a three-step drop, throw. Scrambling five yards is different than running over a wall with 15 pounds of gear strapped to your back. I sure wouldn't want Namath or Elway covering me as I made my dash to a pillbox.
As for the arthritic knee thing, it is possible to jog on bad knees, and exercise has been proven to help the arthritic condition so long as the person starts out slowly and increases the stress slowly. Even though it won't cure the disease, it will increase the range of motion and manipulation of the affected joint. Jogging in t-shirt and shorts at a 15-minute mile pace is a lot different than humping in-country wearing combat boots with a rifle and steel helmet.
Combat training is much more rigorous and demanding on the body than any, and I mean ANY, athletic event other than maybe an epic adventure race such as the Eco-Challenge, and even then, those athletes eat a lot better than soldiers in the field.
Obviously, the writer (a) never served in the military (which he admits) and (b) doesn't participate in endurance sports.