I am more concerned about the physical damage that is done by the constant pounding to the body (other than a heart attack like Fixx). Although, apparently the pounding can affect the heart, too.
Anyway, add 20 to 50 pounds to the body and it compounds the problem. Not that a person who is overweight would be running a marathon. However, they may attempt it and train. Best to lose the weight first.
True there are shoes that reduce the impact force. It helps, but there is still a huge amount of force being applied to the legs, back, and body and continuously for a long period of time. Not good.
One thing that will end an exercise regimen the quickest is an injury. I wish that I had been more careful in my younger days. I have knee issues, too. I remember having shin splints so bad that I couldn't walk up stairs. This is why I chose a bike and hiking. Although I tended to load up my backpack for the day hikes — I believed in “being prepared.”
He's now having trouble with his feet, of all things. The pleasant young podiatrist showed him the x-rays of his feet, and pointed out a certain curvature of his metatarsal bones.
"At some point you've had stress fractures here, and here.", while pointing at each foot.