You're working out too hard. A doctor explains why low-intensity Zone 2 cardio is the key to getting stronger and faster.

• Low-intensity or Zone 2 cardio has big benefits for fitness and health, a doctor said.
• Easy runs can help you improve your endurance and even make you faster.

Stop working so hard — if you want to run faster and go further without getting tired, you might need to slow down first, according to an exercise science expert and triathlete.

Exercising at a low intensity while maintaining a low heart rate — known as Zone 2 training — is an underrated way to build your endurance and overall cardio fitness, said Dr. Morgan Busko, sports medicine physician at NewYork-Presbyterian/Columbia University Irving Medical Center.

"For everyone, the very beginner runner to the most elite athlete, longer runs where you're maintaining a low heart rate are really beneficial for building aerobic endurance," she told Insider. "By running slower for longer periods, you're setting yourself up to be able to run faster."

While not every runner, athlete, or gym goer needs to focus on training in Zone 2, it can be a great tool for making the most of your cardio sessions without overdoing it or risking an injury.

Knowing your heart rate zones can help you train smarter, not harder

The benefit of low-intensity training is that you can actually boost your performance by not pushing yourself as hard on all your workouts.

But to make sense of heart rate zones, you need to understand your maximum heart rate, the highest number of beats per minute your body can handle in a workout.

While the most accurate way to measure maximum heart rate is done via stress testing in a lab, you can estimate it with a simple formula of subtracting your age from 220. For example, a 30-year-old would have an estimated max heart rate of 190.

From there, you can estimate your heart rate zones:

• Zone 1, for light exercise or active recovery, is about 50% to 60% of max heart rate

• Zone 2, an easy pace, for building aerobic endurance, is 60% to 70% of max heart rate

• Zone 3: a moderate pace that's challenging but sustainable, 70% to 80% of max heart rate

• Zone 4: for high-intensity training, 80% to 90% of max heart rate

• Zone 5: near all-out effort that you can't sustain for long is 90% to 100% of max heart rate

As an example, if your max heart rate is 190, Zone 2 training would be between 114 and 133 beats per minute.

The easiest way to monitor your heart rate during exercise is to use a wearable fitness tracker like a smart watch. However, you can also estimate when you're in Zone 2 by aiming for a pace at which you can carry a conversation without feeling out of breath.

How low-intensity or Zone 2 training builds aerobic endurance

A solid understanding of heart rate zones can help runners develop speed and stamina while helping prevent burnout or injury, according to Busko.

Research from elite endurance athletes shows that a successful training balance involves aiming for about 20% to 30% of workouts to be moderate to high intensity — that means about 70% to 80% should be low-intensity or easy runs, she said, a ratio known as polarized training.

"Polarized training is underrated and really important in the world of running because a lot of people think that in order to run a fast time, they need to do all of their runs fast. That's not the case," Busko said.

According to this technique, a majority of your training should be at 60% to 70% of your max heart rate — a good estimate is a pace at which you can easily have a conversation without getting out of breath.

To understand why low-intensity training is so powerful, it's important to know a little about how exercise works to make our bodies stronger.

Aerobic exercise (colloquially known as cardio) occurs when the body has access to oxygen to help power the muscles, like when you're walking, jogging, or biking.

However, once you're moving at a speed or intensity that's too much for the available oxygen supply, you've hit what's known as anaerobic exercise. Examples include sprinting, lifting weights, or high-intensity interval training (HIIT).

For anaerobic exercise, your body starts tapping into stored glucose (sugar, broken down from carbohydrates) as a fuel source. That's important because we store glucose in limited amounts, so once you start burning through it, it's only a matter of time before the tank is depleted.

But the more you train at lower intensities, the longer your body can last before needing to make that switch.

Zone 2 training causes your body to adapt at a cellular level by boosting mitochondria, which increases the power output of the muscle tissues. It also helps train your body to burn fat for fuel instead of carbs.

As a result, your body will be able to handle higher demands and for a longer period of time before getting into the higher heart rate zones and using up its glucose reserves— sort of like building a bigger gas tank.

Zone 2 training can be especially useful if you're doing other exercise or sports

The specifics of how individual athletes should incorporate Zone 2 training into their routines can vary based on factors like experience, total exercise volume, and max heart rate.

The general rule of 80% low intensity, 20% high intensity can help guide you in getting the right mix of running with other exercise, too, according to Busko. If your weekly workouts involve heavy weightlifting or circuit training, easing up on the runs can help you keep building endurance without overdoing it.

"You probably do want to have a fair number of those miles in Zone 2 because you're getting a much higher intensity workout if you're doing CrossFit or HIIT," she said. "

Don't forget to take rest days for best results

The key to Zone 2 is keeping your easy runs easy, and starting to feel exhausted or achey can be a sign you need to slow down, according to Busko.

"If you find that you're not recovering well, you're showing up sore and tired for a run, or you're losing the motivation or zest for it, you may be running too hard,"she said.

But even if you're adding more low-intensity workouts to your schedule, it's still important to prioritize rest days from running, along with active recovery, according to Busko. With her triathlon background, she loves swimming and light biking to give her body a break from running, and also recommends yoga. It's also wise to take a day off from any exercise once a week.

New or less frequent runners don't need to worry about heart rate zones as much

If you're a total beginner to running (or other aerobic exercise) Busko said not to worry about your stats and just focus on having fun with strategies like finding a route you love, or inviting friends to run with you.

"I think there's a lot to be discovered in just the simplicity of running and finding joy in it before diving into all of those metrics," she said.

Unless you're racking up 20 or more miles a week, it's fine to go at whatever pace feels right, according to Busko.

And even experienced or competitive runners may want to take a break sometimes from the stats — Busko said she rarely runs with a GPS watch because it can distract from the experience.

"Giving people too many things to think about is kind of taking away from whether you're enjoying the run," she said.