Your question implies that you have been evaluated and treated...with a diagnosis of 'weak heart' - like the residual after having had a heart attack
or non ischemic cardiomyopathy
If that is the case then I would recommend brisk walking for 20-30 minutes daily. If you can't do that then slow walking or stationary biking or water aerobics
or whatever you can manage. But some version of vigorous activity for 20-30 sustained minutes. Gradually build up as you are able. Perhaps every two weeks you can advance the rate or distance or both. You will have good days and bad days...take advantage of the better days and work a little harder then.
If possible try loosing weight. You would be amazed at how weight loss
will improve your stamina and energy...this goes hand in hand with the above advice of being more active.
Your heart is amazingly adaptive and actually will grow collateral vessels around blocked arteries if made to do so...it doesn't happen over night though. Expect as long a couple of years...but keep at it and you will do well.
, while I endorse it generally, will not strenghten your heart in anyway.