You’ve probably heard some variation of “I hate cardio!” or “I love cardio!” from your workout or running partners. Love it or hate it, training your cardiovascular system requires a little clarity to some key questions: How well does running stimulate that system? And how can running help you improve your cardio fitness? 

Let’s start with some system basics. “Cardiac” refers to the heart, that singular, essential muscle that pumps blood through your body. And “vascular” refers to your pipes—your veins, arteries, and tiny capillaries—which deliver blood pumped by the heart to muscles, organs, and everything else that needs to be sustained with nutrients and oxygen. Put the two words together and you’ve got the cardiovascular system: the pump and pipes working to power and sustain everything you do, including running. 

All of this heart-pumping, oxygen-delivering work goes on constantly, even when you’re just sitting on a couch. But when you further stress this system by running, the system responds and adapts. Muscles, including the heart, get stronger. The pipes become more efficient, adding bigger and more effective lines to power those hard-working muscles.

And while your muscles need oxygen to work efficiently, they can carry on with the work of running, at least for a little while, even when oxygen levels are low. “Aerobic” is the term for that work being done with oxygen, and “anaerobic” is the low-oxygen version. So you’ve got two basic systems, the aerobic and anaerobic, that power your running. (VO2 max scores measure when you’ve hit the top end of your anaerobic threshold.)

Here’s the payoff. Because you are using your aerobic system all the time, even at rest, you never have to worry that you are not running hard enough for a “cardio” (or “cardiovascular” or “aerobic”) workout. Sure, a slow walking pace is not going to provide a lot of stimulation for your body to respond to, but it’s still providing some stress. 

As you progress to a higher level of intensity by increasing your pace from walking, to jogging, to running, and eventually to a full-on sprint, the aerobic demands keep getting higher. Throughout this progression, your muscles start running low on oxygen and shift to doing more of the work anaerobically. But because this type of work is both more demanding and far less efficient than using the aerobic system, runners can only keep up that sprinting-level intensity for short bursts.

Research shows that loads of low-intensity running, like a 40-minute slow jog, has many of the same positive effects as a short, high-intensity workout that may last only 10 or 15 minutes. If you’re time-crunched, you can get a lot of benefits from blasting through a few sets of high-intensity sprints (with a proper warm-up). But on days that you’d prefer a more peaceful, slower-paced effort, be assured that running 20, 30, or 60 minutes at a leisurely pace will provide plenty of cardio benefits.

In fact, even the world’s fastest runners spend loads of time doing low-intensity work to boost their cardiovascular systems (and not burn out, become injured, or just exhaust themselves by doing a lot of high-intensity running).

Stephen Seiler, Ph.D., is a highly regarded exercise researcher well known for his 80-20 guideline, which is based on observing how elite athletes in many endurance sports tend to train. These top athletes often focus most of their workouts (about 80% of total training time) on low-intensity, aerobic sessions. The other workouts (about 20%) may be blisteringly fast, pushing the muscles to tap into that anaerobic system, but there’s no way they can carry on at that intensity all the time.

The takeaway: Elite athletes are a lot like you. They do plenty of workouts that feel relatively easy, and once in a while, they punch it hard. They key is that they, and you, are developing and improving the aerobic system even on runs that feel super easy.

Or, to put it another way, even when it might not look like it, the system is improving when you and they are “doing cardio.”

All articles are for general informational purposes.  Each individual’s needs, preferences, goals and abilities may vary.  Be sure to obtain all appropriate training, expert supervision and/or medical advice before engaging in strenuous or potentially hazardous activity.