When it comes to running training, the focus often goes straight to speed: improving your pace, doing intervals, increasing distance. Or simply, as I used to do when I first started running, pushing myself to the maximum every single kilometre. Yet a fundamental part of running performance is built by running slowly.

Easy running is the foundation of aerobic training and allows you to accumulate kilometres without constantly putting your body under high levels of stress. In recent years, Zone 2 (Z2) has also become an increasingly popular topic among runners and endurance sports enthusiasts. But what does it actually mean, and how much should we trust the data provided by our watches?

Why is easy running so important?

Running at a low intensity helps develop your aerobic base, meaning your body’s ability to use oxygen to produce energy during prolonged exercise. Training regularly at this pace encourages several important adaptations: it improves the muscles’ ability to use oxygen, stimulates the development of mitochondria and capillaries, and makes the body more efficient at using fat as an energy source.

There is also a very practical benefit: running slowly allows us to accumulate more kilometres and spend more time training without constantly building up the fatigue that comes with high-intensity running.

This doesn’t mean that easy running alone is enough to improve every aspect of your performance. Faster workouts are still important for developing speed and the ability to sustain higher paces. The point is that not every run needs to be demanding. By keeping some sessions easy, we can build a solid weekly training volume while having more energy for the harder workouts!

What are heart rate training zones?

Training zones are used to classify exercise intensity. In running, they are often determined by heart rate, but they can also be defined by considering oxygen consumption and ventilatory or lactate thresholds.

Many sports watches use a five-zone model:

• Z1 – Very easy: minimal effort, useful for warming up and recovery.

• Z2 – Easy: aerobic intensity, suitable for easy runs and endurance training.

• Z3 – Moderate: effort increases and talking becomes less comfortable.

• Z4 – Hard: demanding pace, sustainable for shorter periods.

• Z5 – Maximum: very high intensity, mainly used for short efforts.

However, the classification can vary depending on the model being used. In three-zone systems, for example, Zone 2 represents a different intensity compared to Zone 2 in a five-zone model. That’s why we shouldn’t assume that Z2 on every device represents exactly the same level of effort.

Why is everyone talking about Zone 2?

Zone 2 has become very popular because it represents an intensity at which you can train your aerobic system while keeping the effort relatively low. It allows you to spend a long time on your feet, build training volume and stimulate important adaptations for endurance sports, without constantly having to tackle demanding workouts.

It is also a central component of many training programmes based on the 80/20 principle: roughly 80% of training is performed at low intensity, while the remaining 20% is spent at moderate-to-high or high intensity. This is a widely used principle in endurance sports, but it isn’t a rigid formula that works for everyone. The distribution depends on the athlete’s level, goals and how training time is calculated.

The most important takeaway is something else: most of your runs should feel easy, leaving room for more intense and specific sessions.

How can you tell if you’re really running slowly?

This is where my personal approach comes in. I’m not particularly interested in chasing the Zone 2 reading on my GPS watch if it means constantly checking the screen or adjusting my pace every few seconds. For me, the most important thing is that an easy run should actually feel easy.

A simple way to assess this is the so-called talk test: while running, you should be able to talk and speak in complete sentences without becoming breathless.

Your breathing should remain relatively steady, and the effort should feel manageable enough that you could keep running for a long time without becoming exhausted. If you constantly have to interrupt your sentences to catch your breath, you’re probably running too fast for that session to count as an easy run.

Of course, the talk test is a practical guideline rather than a precise measurement of your physiological thresholds. But it can be extremely useful for managing your everyday runs.

How reliable are the zones on your watch?

Many smartwatches automatically calculate heart rate zones using parameters such as maximum heart rate, resting heart rate or an estimated threshold. The problem is that these values don’t always reflect individual characteristics.

If your zones have been calculated using a theoretical maximum heart rate, for example, they could be quite far from your actual zones. Wrist-based heart rate measurements can also be inaccurate, particularly when exercise intensity changes rapidly.

To get a better understanding of your personal zones, you can take an exercise test with respiratory gas analysis or a specific test to identify your thresholds, carried out with the support of a qualified professional.

There are also simpler methods that can provide useful indications:

• Talk test: check whether you can comfortably hold a conversation while running.

• Breathing test: observe whether your breathing remains steady and controlled.

• Heart rate test: compare how you feel during easy runs with the heart rate readings recorded by your watch, keeping in mind that heat, fatigue, dehydration and other factors can influence these values.

If you want to identify your training zones more accurately, my advice is to consult a specialist rather than relying exclusively on your watch’s automatic calculations.

My advice: don’t be afraid to slow down

Easy running is essential, but sometimes it’s the type of training we find hardest to stick to. We’re used to looking at our pace, comparing times and trying to improve with every run. As a result, even an easy session can end up becoming a workout that’s too intense.

Personally, I strongly believe in the 80/20 principle and try to keep my easy runs genuinely easy, without worrying too much about the pace displayed by my GPS watch. But that doesn’t mean I rigidly follow the heart rate zones on my watch.

For example, I can’t run in my Zone 2 without changing what I consider to be my natural running stride. When I try to keep my heart rate within that zone, I end up with a less fluid running style, mechanics that don’t feel natural to me and more pronounced vertical oscillation. As a result, my easy runs tend to fall within the lower end of Zone 3 according to my watch.

I don’t know whether this is due to my watch’s zones not being calibrated accurately or simply because the values don’t precisely reflect my physiological characteristics. However, there’s an important distinction to make: being in Zone 3 according to your watch doesn’t necessarily mean you’re running at a moderate physiological intensity. If your zones are incorrectly calculated, the classification of your effort can also be misleading. To understand which zone I’m actually running in, I would need to identify my thresholds through a specific test.

What I do know is that I prefer listening to how my body feels, paying attention to my breathing and maintaining a natural running stride rather than blindly chasing a number for its own sake.

This doesn’t mean ignoring heart rate or training guidelines: they are useful tools, especially when you know your personal zones and understand how to interpret them. But they shouldn’t completely replace how you feel or the quality of your running mechanics.

You don’t have to prove how fast you can go in every training session. Slowing down when necessary, being able to hold a conversation and finishing a run feeling as though you could keep going are often much more useful indicators than a number on your watch.