I suggest you do things that are enjoyable for you, and concentrate on getting mild to moderate exertion - and forget about heart rates and such for now. Let your body tell you when it's working at a comfortable level, and don't push to the point of numbness or extreme muscle fatigue. If you build up a habit of regular walks or biking, your legs with start to catch up to your heart. Look into some bodyweight exercises for strengthening them, but mostly what you need is to build up muscle endurance.
Sounds like you're out there kicking ass and as long as you listen to body, you can do whatever you like!