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Science says you should run slower to improve cardio—here’s what happened when I followed that advice for a month
I used to take lots of shortcuts when training for a race.
Often those shortcuts involved brief interval sessions and hill sprints, and very few long, slow and steady runs. My theory: the faster I trained, the quicker I’d move on race day.
Over time, and having interviewed countless endurance experts, I’ve come to learn that I’ve been getting it all back-to-front.
Apparently, easy runs, the ones I’ve been avoiding, are equally—if not more—integral than any other form of running session.
Running slower, I’ve been told, is the antidote to a common running mistake.
Specifically, training in zone 2 is said to be the sweet spot if you want to boost aerobic fitness.
With an ÖTILLÖ swimrun in Utö, Sweden on the horizon, I resolved to put this to the test by performing at least two zone 2 training sessions of at least 30 minutes per week for 30 days.
Here’s what I learned along the way.
What is zone 2 training?
While it’s fashionable to talk about zone 2 training in running circles, it’s equally applicable to walking, swimming, cycling and any other form of cardiovascular exercise. In very simplified terms, it’s low-intensity cardio.
It can get complicated if you let it, but one straightforward way of thinking about it is by using five heart rate zones. Zone 1 involves the lowest intensity and zone 5 the highest.
- Zone 1: 50-60% of max heart rate
- Zone 2: 60-70% of max heart rate
- Zone 3: 70-80% of max heart rate
- Zone 4: 80-90% of max heart rate
- Zone 5: 90-100% of max heart rate
As you move from the lowest to the highest zones, your body will switch from using stored fat as its primary fuel source to carbs and protein.
In terms of heart rate zones, zone 2 training approximates to exercising at an intensity that maintains your heart rate within the 60-70% bracket.
You can monitor your heart rate using a sports watch like the Coros Pace 3 or the Apple Watch, although you absolutely don’t have to and you shouldn’t call a zone 2 workout a bust if your device tells you that you’re working at 72% of your max heart rate for a bit.
Smartwatches like the Apple Watch Ultra will show you which heart rate zone you’re in during your run.
(Image credit: Future)
You can also use the rate of perceived exertion (RPE) scale, aiming for a 5-6 out of 10, or Andrejs Birjukovs, a certified endurance coach, former professional athlete and author of The Athlete Blog, recommends the talk test.
A sign that you’re exercising in zone 2 would be that you could “read out loud a 30-50 word passage without issues and breathe freely through the nose,” says Birjukovs.
The benefits of zone 2 training
Performing more zone 2 training—rather than neglecting it entirely as had been my previous approach—triggers a raft of beneficial adaptations at the molecular level.
“Physiologically, zone 2 training takes place at the point where lactate, a byproduct of energy metabolism, starts to slowly accumulate and overall fatigue is minimal,” says Birjukovs.
It’s also the point where fat oxidation is at its highest. “Combined, this improves the body’s metabolic capabilities.
“It increases fat oxidation and glucose control. It increases the stroke volume of the heart and overall blood plasma volume.
“It improves the capillary network and builds mitochondria around the muscles that help deliver energy quicker and more effectively.”
Regular zone 2 running also improves muscle strength, along with bone and tendon resilience.
“Practically, this level of intensity allows the body to adapt gradually to the increasing workload, making it stronger, more durable and efficient,” says Birjukovs.
“The difference between an elite athlete and the beginner comes down to how much workload or force the body can overcome, and zone 2 training helps to build that capacity over years.”
My experience
1. I didn’t suffer any injuries
Gladly, with the money and time I was set to invest in getting to and from Sweden, I didn’t get injured.
This was partly because I swam half of my zone 2 training sessions and partly because the running workouts were performed at a lower intensity than I was accustomed to.
In fact, I pulled a muscle in my back the previous month from an overzealous interval run and full-body workout, so the lighter pace was very welcome.
At both the everyday and elite level, easy runs tend to be favored to boost performance for this very reason.
“Zone 2 training is something that can benefit everyone,” Kaleigh Ray, an ACSM-certified exercise physiologist and run coach tells Fit&Well.
“By keeping the bulk of your runs easy, you improve your aerobic capacity while minimizing your risk of injury.”
To underline her point, Ray notes a 2021 study comparing the merits of easy runs, tempo runs and interval training.
“This study found that easy runs, which contributed the most to total training volume, were the most important in determining performance outcomes,” says Ray.
“Since the research shows that short and long interval runs contribute the least to performance, it makes sense to minimize the number of miles per week spent doing these types of workouts.”
Easy runs, on the other hand, let you run more frequently while allowing for better recovery.
“If that leads to fewer injuries and more consistent training, that’s beneficial for runners at any level,” says Ray.
2. I learned a new skill—nasal breathing
When swimming, maintaining a zone 2 pace was relatively easy, but I found it surprisingly tricky when setting off on a run.
I was using my Apple Watch, aiming to stay between 130-140bpm, but I’d often have to slow myself down to avoid shooting too far into zone 3.
Then I remembered advice from running coach James Stirling.
“Nasal breathing involves exclusively inhaling and exhaling via your nose during easy runs. It’s quite a good way to pull you back from pushing too hard,” he previously told Fit&Well.
It took a bit of getting used to, but it was an effective way to throttle back my pace and gave me something else to think about rather than just checking my watch every five seconds.
Want to give nasal breathing a try? Stirling suggests using a run-walk protocol to get to grips with it. “Start off simply by walking, inhaling and exhaling through your nose, then gradually start introducing gentle running as you get used to it,” he said.
(Image credit: Getty Images / NoSystem images)
“You don’t want to be panting really hard. If someone sounds like a steam engine coming past you, that’s too much.” Instead, he says, adapting to nasal breathing requires a gradual introduction.
“It’s something that can be learned and the body can be trained to do, but it does take a little bit of patience and perseverance to get used to.”
3. My VO2 max didn’t take a hit
I expected some change.
Either that doing more zone 2 training would notably boost my VO2 max—a measure of the amount of oxygen your body can use during exercise and a key indicator of overall fitness—or that doing fewer hard interval sessions would see it plummet.
Instead, it remained steady at 52 mL/kg/min from the start of May through to mid-June.
For Cassandra Padula Burke, a RRCA-certified running coach and owner of Catalyst Performance Lab, that should come as no surprise.
“Zone 2 training helps endurance athletes build a base of fitness to perform faster, for longer durations and with less fatigue,” she explains.
In part, this is because training at this intensity with the availability of oxygen helps create more mitochondria in our muscle cells. “More mitochondria means faster recovery, better energy efficiency and the ability to sustain longer duration training and racing,” says Burke.
“Most of an endurance athlete’s training should be spent training in zone 2. It’s not flashy, but it will provide the most significant long-term performance gains.”
4. I completed my swimrun
Crucially, swapping out hard interval sessions and hill sprints for more easy runs got me to the start line and through to the finish in one piece.
There were times, especially when scrambling over rocky trails on the back end of the course, when my legs felt like they might fail me.
But my ticker kept ticking along and I was able to sustain a sprint finish when I knew the last swim leg in the chilly Baltic Sea was well and truly behind me.
How to start with zone 2 training
(Image credit: Getty Images / LSOphoto)
If you want to integrate more zone 2 training into your weekly workouts, Birjukovs recommends starting out with one of his favorite run-walk drills: Alternate three minutes of running with two minutes of walking.
“Instead of doing a 40-minute run and gasping for air throughout the entire time, split it into blocks of a three-minute run and a two-minute walk,” he says.
“At the end of the session you’ll feel much fresher, be more willing to repeat it the next day and have capacity to grow the volume over time.”
That consistency, he adds, is what will get you stronger and faster over time.
