Any type of cardio will be good for heart health. If you're trying to get the best bang for your buck time wise, I'd go with HIIT.
HIIT is very beneficial at helping increase V02 Max and metabolic rate, and it doesn't take up a ton of time. I personally do some HIIT, and I also enjoy hiking or walking outdoors for cardio. I stay away from jogging and long distance running for personal preference, but it is good for the heart as well.
I would start by adding in 2-3 sessions per week if you're looking to improve your cardiovascular health. It doesn't need to be anything extreme. And as stated above, account for the calories burned from cardio by eating more so you don't start losing weight.