Like jamj1974 I’ve had a new aortic root and aortic valve (mechanical) but had mine done 12 weeks ago. I’m 52 and am well on the way to full recovery. I’m being told to exercise as much as I can so I’m trying to get out and walk/hike as much as I can (not as easy now I’m back at work) and can manage about 4 to 5 miles of Lake District walking, so fairly steep hills. I also go to the heart rehabilitation gym twice a week for supervised workouts.
I did have to go through about a year of no exercise (not even on an ebike) and no heavy lifting so I know what you’re going through. Before my operation I would get out of breath quickly, even on easy walks.
Try an ebike if you’re allowed. I had one for about a year and with minimal/zero effort effort you can keep up with your mates and get out riding. I found it easier than walking. I just set it on turbo mode and span slowly to keep my heart rate as low as possible. Now I’m fixed I plan to switch back to regular bikes once my chest heals and my strength returns.
I used to ride off road with a chap who had a quadruple bypass when in his late sixties and was back out on his mountain bike within a few months. No doubt your heart problem is fixable and you’ll be better than ever afterwards. You just have to keep your spirits up while you wait.