I a hoping you will put far more importance in your doctor's advice than in any post you see here (or in any internet forum). I am sure that the person who responded to you means well, but your doctor has examined you and nobody here has.
It is correct that the heart is primarily muscle (a much different sort of muscle than other muscles in the human body, different type of muscle cells altogether), but it is the only one that has to contract to keep us alive, every minute of every day we live from before birth to the last heartbeat of our lives when we die.
One mistake could be fatal. Please spend more time talking to your doctor (second opinions from another doctor can be helpful too, just be sure the second doc gets all the info and data that the first one has gathered) in depth about YOU and exercise, weight liftting in particular.
I am 32 and had two heart attacks last year. I have a hereditory disease where the right side of my heart is buggered.
Anyway, they told me not to go to the gym anymore but a guy I was in hospital with told me they said the same thing to him a year earlier and he went to the gym and excersised regularly and when they scanned his heart a year later they couldnt believe it was getting better.
The fact is that the heart is a muscle and like any other, train it and it will get stronger. They told me dont go to the gym but my wife is a fitness instructor and told me to keep excersising so after three months of recovery I began light wieghts. I now push more than I did pre heart attack and am in much better shape!
Keep excersising but not so much cadio, do a warm up with cardio of maybe power walking but no running then do some weights but know your own bodies limits.
If you need to speak to my wife to chat about what to do give us a shout and I will get her to mail you.
Good luck