Thank you for asking your query.
I assume that the tests - echo , angiogram , the 64 CT etc has been done recently. In that case the culprit for the limited aerobic capacity may not be your heart at all.
There are some other possibilities I would consider as a cause of this. One reason could be your body is not acclimatized to exertion. This can happen if you have been sedentary for a while and then restarted the exercise. The workaround is the do aerobic activity for tolerable duration and then increase it by a couple of minutes every day.
Do not exert to the point where you are gasping for breath. Another cause could be in your lungs. It could be an exercise variant asthma or hyperactivity of the airway. In that case all the imaging tests will be normal. You may need a pulmonary function test to find the exact cause of it. The third possiblity I would consider is a Thyroid hormone related problem. A hypothyroid state can make you easily tired. I recommend a Thyroid function tests.
I assume that you do not have any other symptoms like chest pain or swelling of legs. In that case its prudent to rule some non cardiac causes of breathlessness as told above. That said, there could be a cardiac element for this breathlessness. Since there is an 30%LAD, it could have affected the cardiac walls. It could be too early to show in an Echo. As a part of the standard care you will need periodic Echo monitoring too.
Hope this helps