Maggie's Farm

There are sort-of 2 things that people term "cardio" exercise. One is about endurance (eg swimming laps, mountain hiking, treadmill jogs, etc). Maintaining endurance is good, but it has little to do with heart fitness unless one has cardiovascular disease or some other form of heart ailment.

It requires intensity to properly stress the heart muscle. Like weight-lifting, the intensity is what demands muscle strengthening and improved vasculature. This does not take much time out of your workout schedule. Heart rate is a good measure of exercise intensity. Here's how to know your cardio stress level. (It might not exactly apply if you are taking blood pressure meds that limit heart rate.) Or you can go by how you feel - if you are winded by a one-minute sprint and need 30-60 seconds to catch your breath, you probably got your heart rate up.

Here's a good 10-minute cardio workout, with jump rope. He calls it Beginner but it is pretty good: