It's been 15 months since I had two heart valves replaced with mechanical clickers. I wrote several posts about this. If you're interested in reading them, the links are below.
What I want to say is that I'm okay now. More than okay. A week ago I went snowshoeing--2 hrs/day, a few days in a row. Strenuous? Some of it was straight uphill. Yesterday I walked 12 k (7.5 miles). Once the weather is nicer, I'll start cycling again.
Indeed, it's possible to have heart surgery and get back to an excellent level of activity--assuming, of course, that's what you want. I'm still fairly young (mid 50s) for having had heart surgery. I was easily fatigued in the months immediately after surgery. I had to push myself. My heart often felt like it was pounding fit to burst. I told my cardiologist who said that my body had to get used to having blood coursing in a way that it hadn't for years. I had to believe that made sense (which it did) and that I wasn't hurting myself by exercising.
Before I had my surgery, I did some research and discovered that there is a non-psychotic mental disorder called Skumin's Syndrome. What can happen--ie it doesn't have to--is that people who've had heart surgery are afraid to push their heart; they constantly check their pulse; they're anxious; the persistent clacking of their new valve disturbs them and keeps them awake at night. GUESS WHAT? YOU GET USED TO IT. You do. You think you won't, but you do. It doesn't happen in the first two months, nor even the first ten months. But you do.
As for the anxieties that characterize Skumin's Syndrome? Trust your doctors. The surgery was done to correct a problem. It's corrected now. Give yourself time to heal. Start to exercise. Go live what's left of your life.