Though I had an ascending aortic aneurysm, my story is much like yours. I too had open heart the aneurysm was discovered accidentally as I had no symptoms. It was about 6 cm. Had the surgery May 10, 2016. I did the cardiac rehab program at Mayos suggestion in my home town. After that, I have tried to walk at least a couple of miles each day. I have had no problems with any exercise and know I should do more. Really have had no aftereffects from the surgery. If I were you, I would ease into excercise, perhaps talking to your doctor about it, and then try to increase your time spent exercising. As I am now 75, I find I need to do some kind of fitness program to keep from being stiff. Hope this helps a bit and the very best to you.