Love it or hate it, there are few other exercises that elicit as powerful a cardiovascular response as running. This is because you’re engaging all your major muscle groups, particularly the large muscles of your lower body, to carry your entire weight over the course of several miles. That's hard. To maintain a steady stream of fuel to your working muscles, your breath rate and heart rate increase to pump more oxygenated blood to your working muscles.
This is the definition of a cardiovascular workout.
But if you’re sitting there thinking, "There is no chance in hell I'm about to run for 30 minutes straight," don't worry, no one's asking you to. High-intensity interval training (in this case, interval running) is every bit as good at improving heart health as slower, prolonged, steady-state exercise. All you have to do is alternate between periods of high-intensity running and lower-intensity walking. Because you'll increase your heart rate significantly during the high-intensity intervals, your heart rate will remain high even during the lower-intensity intervals as your body works to recover. So while mentally and physically you give your body a break, physiologically you continue to work.
If you're just starting out, try a 1:2 ratio of all-out effort to low-intensity work. For instance, you might run as fast as you can for 30 seconds, then give yourself 60 seconds to recover before your next running interval. As you feel yourself improving, you can graduate to a 1:1 ratio (for instance, 30 seconds of work to 30 seconds of recovery time), or even a 2:1 or 3:1 ratio of work to rest. And remember, the key words here are to run "as fast as you can" -- depending on your fitness level, this may be the equivalent of a fast walk. Trust me, that's OK.