Thank you for your question.
A couple of possibilities come to mind. Aside from coronary artery disease(ruled out by your echo, there are other medical conditions that can cause your symptoms.
First of all, anemia is a common cause of poor physical endurance and shortness of breath. If you have not had your blood count checked within the past 12 months, you should ask for the test.
) diseases can cause easy fatigue and shortness of breath. A chest xray should be done to rule out underlying lung disease.
Thirdly, if you have leg pain with walking/exertion, the arteries in your legs may be unhealthy(peripheral artery disease). This condition can be treated with a blood thinner like aspirin, as well as medications to dilate the blood vessels in your legs.
Finally, if you are just starting an exercise program, it is possible that you are simply out of shape. You can increase your endurance as you exercise on a more regular basis.
I hope this is helpful to you.