It's not as much about your "experience". IIRC there was scarring in heart tissue after running a marathon distance in a large majority of runners independent

It's not as much about your "experience". IIRC there was scarring in heart tissue after running a marathon distance in a large majority of runners independent of training. This could plausibly impede blood flow if the heart's fitness dropped back to "normal" (i.e. in old age). That said, I still do distance running.