A weak heart can be expressed by determining an individual's EF (ejection fraction...the amount of blood pumped into circulation with each heartbeat). Less blood is pumped due to heart muscle damage and a significantly enlarged left ventricle will also decrease contractility.
It is possible your heart is enlarged due to the stress of the operation. A weakened heart that has less than 29% EF is usually not subject to surgery unless it is an emergency, and an EF less than 50% is not a very good candidate and sometime rejected for surgery until the condition improves.
To help get a prognosis, it may depend on your condition prior to your surgery. But to answer your question can one increase their EF? Yes, my EF was below 29% (heart failure range) 6 years ago, and currently it is pumping in the normal range. There are others on the forum that have increased their EF.
The surgeon said there would be an improvement in about 6 months, so that indicates there is no heart cell necrosis that would permanently impair contractility and your heart needs time to recover.
Heart medications can help strengthen the heart. My father after being on Lanoxin for 10 years now has a strong heart beat where it was so weak before that I could hardly feel his pulse in his wrist. Studies show that exercise does help strengthen the heart muscle even with people who have cardiomyopathy or congestive heart failure. It was previously thought that exercise did not improve the heart muscle, but recently I read in consumer reports that it does help. If your doctor said that your heart muscle can improve, that is a good sign. Make sure that you are breathing correctly and not taking in a ton of air in through your mouth. Sometimes people feel like they are smothering and panic taking in a lot of air and consequently hyperventilating. This happened to my mom and she had to carry a brown paper lunch sack everywhere she went and to bed at night. I have heard many good things about the pig valve (no rejection and lasting a long time). I even know a pilot that was allowed to fly his own airplane after having a pig valve. It takes most people a year to recover from open heart surgery, so be patient. It is great to hear that your surgery was a success. A lot of people are not that fortunate. Surgeons told us it is a 1 in 5 chance of not surviving open heart surgery for everyone. When I was in the hospital surgery intensive care unit when my mom had surgery, I only seen one person survive and leave surgery intensive care to be moved to intensive care. Many people did not make it in the three weeks we were there. This was at one of the best cardiac hospitals and took in mostly emergency high-risk patients. Take your medication as prescribed by your doctor. Try a brown paper bag if you hyperventilate. It may be worse when getting up out of a car, so be prepared for breathlessness then, which can be extremely severe. Also, in the cold keep your mouth and nose covered. Cold air makes it worse. Take care.