"And so, I've been following the instruction from cardiac rehab only. 45 minutes a day of treadmill or bike but it is not kung fu"
So that was cardiac rehab 4 years ago. I'm assuming that by now your MD has cleared you to resume normal activities. "Kung Fu", by definition would not constitute a "normal activity" for the average person.
My question is: what are your physical restrictions if any? is it ok for your heart rate to exceed a certain amount of beats per minute? Are you to avoid impact to the chest and abdomal region? Can you engage in hard physical compression with another student, or by forming a specific posture such as in yoga?
Most all of us middle age and up, older warriors have one or more types of physical restrictions due to repeated orthopedic surgeries [like myself] and/or other aliments pre existing or newly emerged. Pain in the joints rein supreme. Just ask anyone in that age group, "What is the pain of the day?" And you'll get a litany of complaints. Ultimately as time passes we learn how to work around these limitations, get a good workout and move forward in the pursuit of our martial skills and life in general. Don't let an off day where the "Chi is low", send you into a depression.
Furthermore, the onus for health and well being rests on YOU and your choices more than it does on your instructor. You know [or should anyway] your body better than anyone. Tell your instructor exactly what your limitations are and if there is something you don't feel comfortable with DON'T DO IT. Simple as that.
Pick a style that is age, health and wellness appropriate. Ie: not MMA, Tai Boxing etc. A martial art should build up and restore- not tear down. Even BJJ practiced conservatively keeps Sports Medicine personal in business. You're on point with Tai Chi and swimming. It doesn't get any better than that. I see 90 year old's in the pool who can barely walk, swim through the water with ease.
"Fortify the strengths and strengthen the weaknesses while you build up a reserve", thats my personal philosophy. Move forward buddy. Enjoy and appreciate the present moment....because thats all we really have!
Buy the best and cry once!