I am in an organized, hospital-administered cardiac rehab program, which if such a thing is available, is way safer, IMO, than exercising that much on your own. As another person mentioned, it makes a big difference whether you are on a Beta Blocker or not. Also, if you had any left ventricle hypertrophy secondary to the valve problem, (as I did and do), both my surgeon and cardiologist made a point of saying that in order to allow the ventricle to improve its contractility - assuming it can - the way to ensure that is not push the heart too hard too soon. Since your signature mentions you had both severe stenosis and regurgitation, I would be willing to bet money (if I had any, LOL) that you are really, really overdoing it.
Does your medical facility, where you had your surgery, have any organized exercise program? Even though you mentioned being fit before, do not assume such a program is only for couch potatoes and wimps. Because such programs (at least in this country) include issues such as target heart rate or RPE if on Beta Blockers, EKG leads to monitor HR, Resp rate and periodic blood pressure checks, it is the safest way to make sure the heart is being gradually strengthened, not overworked. That actually was kind of hard for me to understand at first, as I felt so much better with the new valve in place that I had not realized that there is a lot more involved in the heart's recovery than just valve function. I hope that helps!
Regarding the RPE (perceived exertion scale) I was instructed that with my having had same problem you had, stenosis and regurg, and now being on a Beta Blocker, that they want my exertion rate to not go beyond a 13. Thus my target range is 11 - 13 during exericse at this time. Today I found my treadmill setting to be too easy, so they told me just to bump up the speed until I felt as if I was in the target range. I did and do find it very helpful to have that method of gauging exertion.