Even if you did ruin your knees from running (or just had a knee injury in general), it's not as though you would magically balloon up over night. You'd just go find a new activity and continue to watch what you eat. Fats are just full of excuses. There are plenty of people who have bad knees, a bad back, bad hips, joint pain, or genetic heart conditions or the like.. doesn't make you fat. What does make someone fat is stuffing their face and pretending that there's no physical activity they can do.