If your heart tissue is damaged, it has become scarred and is now probably quite 'stiff' in one of the chambers. Your heart has become enlarged very likely due to the exercise. As we age, our hearts do stiffen and have to work in a different way to young people. In the young, the heart muscle is very elastic and when more oxygen is needed in the body due to exercise, it only has to squeeze harder to get more blood ejected from the
chambers. Due to its elasticity at a young age, it can easily relax quickly enough when
working fast to fill up in time for the next phase. Older people don't have this luxury and the heart tends to enlarge, to allow a larger volume of blood to be ejected with each beat.
If there is tissue damage, depending on how much tissue, this can cause the heart to enlarge even more to try and compensate. Lowering your exercise to just a 30min walk
each day should be sufficient to stay fit. I am glad that you don't have any severe symptoms. Someone else can jump in hopefully now and correct me if need be because
I don't believe there is anything that can be done to correct scarred heart tissue.
The worse case scenario, which doesn't happen to everyone in your situation, is that the
chambers can go out of shape and become even less effective. In some people this never happens and the heart merrily works for the rest of their natural lives. I take it that you suffered a bad heart attack at some time?