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[PrissyPro]

The couple of people I know who have heart problems they mostly do 10 minute walks throughout the day. For example when they first get up in the morning, when they get home from work or school, and probably an hour before bed. There are many walking videos on Youtube. There are also many short 5 minute strength workouts there (with and with out weights) . You don't have to do anything super intense, the younger you are the quicker and easier it is for you to lose weight. So do not stress out about it.
For the floor crunches being to hard, you can do standing crunches. Even better you can do planks with your knees down (just like 32spartan11 said).

Lastly check out chair workouts. Yes I know mostly seniors do this type of workout, but it seems to work.