your body has three muscle types - fast twitch, slow twitch, and smooth.
fast is for sprinting and lifting heavy shit.
slow is for endurance (the red meat on animals).
and smooth muslce is the involuntary muscles such as your diaphragm, heart, and your various sphincters in your system.
steroids will attach to receptors in the fast and slow twitch muscles - they won't do jack shit to your heart.
they are shown to raise your blood pressure and perhaps lower the good cholesterol in your system while you are on (both of these fix themselves once you come down). due to these, it would be good for you to do cardio - it strengthens your cardio/pulminary system (heart and lungs).
also, the only negative thing I can say about cardio is that you can force your body into a catabolic state and lose muscle mass - but that is if you are natural.
one of the good reasons to use steroids is that you can lift and do cardio and it won't hurt your gains like it would if you are natural.
as for organ growth and heart enlargement, you might be getting steroids side effects mized up with extended use of Growth Hormone.