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How running slower makes you faster

It’s true: slow runs help make you faster on race day. We asked elite coaches and athletes to explain why.


If you think elite distance runners are spending every training session pushing themselves at superhuman paces, think again. In fact, while the mileage a pro puts in the bank each week is out of reach for most of us, much of this volume is done at paces that sound distinctly, well, human.


Why so slow? We asked world-class On athletes and coaches to explain how increased mileage at slower speeds can make you faster – and how we can incorporate it into our running regimes. Before we get to their run-faster recommendations, however, we need to understand a bit of sports science 101 –  the difference between aerobic and anaerobic training. 


Aerobic vs. anaerobic training

Aerobic activity is defined by the American College of Sports Medicine (ASCM) “any activity that uses large muscle groups, can be maintained continuously and is rhythmic in nature.” The key word in the definition as applied to running is “continuously."


Aerobic running is easy running at below 80% of your maximum heart rate. Running in this aerobic zone maximizes an athlete’s ability to burn fat as a fuel source. It’s running at the kind of pace where you think you could go forever.  In the aerobic zone your body uses oxygen to power the muscles, fuelling them with both glycogen (carbohydrate stored in the muscles) and fat (its preferred energy source). With this power cocktail of fuel, your muscles can keep going. And going. And going.  


Anaerobic training is what happens when you kick the intensity up a few gears. The ASCM defines it as “intense physical activity of very short duration, fueled by the energy sources within the contracting muscles and independent of the use of inhaled oxygen as an energy source.”


In plainer running terms, we are now out of the comfort zone and into the hurt locker at a pace we know we can’t sustain for very long. 



When we get to above 80% of our maximum heart rate, we enter the anaerobic zone. Here, we stop using oxygen to power the muscles and without oxygen we can’t burn much fat. This means, as stated in the above definition, we rely on the energy stored within our muscles. The primary energy source within muscles is glycogen and our glycogen reserves are used up much faster than our fat stores. At such high heart rates, glycogen is less efficient and yields less energy. Even fully topped up with pre-race pasta party carb-loading and good breakfast, our glycogen stores will only power us for less than two hours. If you’re running a marathon, that’s only enough if you plan on breaking the world record. 


When you push yourself into the anaerobic zone then eventually you’ll run out of glycogen. This will not be pretty. Running out of glycogen is what we all know and dread as “hitting the wall” or “bonking.” Entering an anaerobic state is also the point where lactic acid really starts to build up in the muscles. This biological mechanism stops you pushing your muscles beyond their limits by slowing you down. It’s a clever evolutionary trick, but it’s not going to help you get that new PR you were hoping for. 


The key outtake here for long distance runners is that we want our bodies performing in the aerobic zone for as long as possible on race day. Which means we need to train our bodies in the aerobic zone so we can go faster by burning fat without gobbling glycogen. You can probably see where this is going…



Easy days easy, hard days hard

Chances are the pace of your regular run lands somewhere right in the middle of “this feels easy” and “I think I’m going to pass out.” That point is the boundary between the aerobic activity, which you can do for a long time, and the anaerobic activity, which you can only do for a short time. But what might feel like your sweet spot is considered by many coaches to be more of a tempo no man’s land in between the two activity types. 


While there are certainly benefits to getting out there no matter the pace, the respective training benefits of aerobic and anaerobic training come more efficiently when you focus on one or the other in each session. As the old adage goes, run easy days easy and hard days hard. This means running most of your miles at easier paces, interspersing this aerobic training with some hard, out-of-the-comfort zone efforts (more on that later). 


“Our elite marathoners typically run 85-90% of their training volume in the aerobic zone,” says Andrew Kastor, Head Coach of the Mammoth Track Club in Mammoth, California, US.  


It’s a similar story over at the On Zap Endurance team based out of Blowing Rock, North Carolina, US, according to Elite Athlete Coach Pete Rea: “During marathon preparation roughly 75-78% of our athletes’ total weekly volume is run at paces slower than target marathon rhythms.” 



How slow should you go? 

So what is the right pace for your aerobic workout? A tried and tested way of staying in the aerobic zone is to keep your heart rate at no more than 80% of your max while you run. Most smart watches nowadays will do these calculations for you, defining clear zones, but you can also do the math yourself provided you have a heart rate monitor.


To estimate your maximum heart rate, simply subtract your age from 220. E.g., A 30-year-old would have a max heart rate of 220-30 = 190 beats per minute. 


Once you know your max you can work backwards to determine your heart rate zones. When you want to improve your aerobic performance you need to stay in the first three zones defined below. 



Applying these HR zones to pacing recommendations is difficult, but as a guide, it is usually at least 90 seconds per mile (56 seconds per kilometer) slower than your target race pace.  For non-elite athletes, coach Luke Humphrey, the main proponent of the famous Hanson’s Marathon Method, recommends 1:30–2:30 minutes per mile (around 1:00–1:30 minutes per kilometer) slower than goal race pace. At the faster end of this range, you’re likely in HR zones 2–3. At the slower end, you’ll be in HR zones 1–2. 


While the main thing is to ensure you’re not creeping into the anaerobic HR zone 4, it can be good to vary the pacing of your aerobic runs. On the day following a hard aerobic session, for example, staying in Zones 1–2 will support recovery. Other days, if you’re feeling good, Zone 3 running is going to bring solid benefits. 


Creeping into low Zone 4 is not where you want to be – best to save those anaerobic efforts for the really tough workouts where you’ll be firmly in Zone 4 and maybe Zone 5 for short periods. If you’re interested in calculating training paces for the different sessions, The Hanson’s Method Calculator is a useful tool. 



The frequency and distance of your aerobic running depends on your goals, of course, but a good guide is provided by the pro coaches above. They prescribe up to 90% of the total training volume as aerobic for their marathon runners. It’s worth noting that while this would not be recommended for sprinters, any runner targeting a race distance of over 5 km can apply this philosophy with great effect. Of course, the longer your goal distance, the longer the LSD (Long Slow Distance) runs you should work into your plan. As another rule of thumb, and in line with the Hanson’s Marathon Method, two anaerobic workouts a week are the absolute maximum, and at least one day a week should be dedicated to complete rest or low-impact cross training like cycling or swimming. 


The benefits of aerobic training

If you’re new to focused aerobic training, making slower paces your go-to may seem too good to be true at first. If you’re used to just running by feel, you’ll likely have trouble keeping the lower heart rate zones and be tempted to pick up the pace. 


However, as 2018 Ironman World Championships runner-up Bart Aernouts (pictured) said when stopped by On HQ recently, “a slow run can only be too fast, not too slow.” Bart’s really easy runs are paced between 6:54/mile (4:17/km) and 8:03/mile (5:00/km). That’s not super slow, but some of you are likely running most of your runs at a similar pace – and you’re likely not one of only two men to finish the iconic Kona Ironman course in under eight hours, running a 2:45:41 marathon, averaging 6:19/mile (3:56/km) after a 2.4-mile (3.86 km) swim, and a 112-mile (180.25 km) ride in the Hawaiian heat. If that’s the case, it’s time to slow it down. 



Bart’s philosophy is that doing more work at lower intensities really benefits you in the long run (pun intended). While others might be training at a higher intensity more regularly, they also get injured more often. And while they are recovering, Bart is still training, leading to a higher volume overall. 


And that’s just the start of the list of benefits that should lead you to make slow runs your go-to training method. 


“When running in the aerobic training zone, an athlete’s ability to burn fat as a fuel source increases and becomes optimized,” Coach Andrew Kastor explains. “Fat utilization during running plays a major role in completing a marathon at a desired goal pace.”  


“At the cellular level, slow aerobic running develops aerobic enzymes and mitochondrial density which helps increase aerobic energy production.” 


“It also builds resistance to fatigue so you become durable to the long grind that is marathon training. Last but not least, it also allows an athlete’s mind to relax and enjoy the pleasure of the sport.”


But as Coach Pete Rea points out, while slower training is a big part of optimized endurance training, it’s not the only element. 


“Easier running allows athletes to recover from harder workouts and continues to strengthen connective tissue and expand capillary bed production throughout the build up. From a mental standpoint as well controlled aerobic running lets marathoners rejuvenate between those tougher more specific sessions.”


A winning combination

There are also huge fitness benefits to be had from the more focused speed sessions Coach Rea refers to – remember, easy runs easy but hard runs hard. Hard sessions commonly take the form of interval workouts that push you firmly beyond your aerobic threshold and into the anaerobic zone, aka “the pain cave.”  Anaerobic workouts build strength and explosive power that you want to have in your arsenal, especially as you cruise into the final stages of a race. 


If you’re interested in learning more about anaerobic intervals, check out our introduction to interval training. But before you do, take your time to absorb the message here: that adding more easy runs could be the key to unlocking the endurance and speed you need for a new personal best, all while enhancing recovery and limiting the likelihood of injury.  As backed up by science and proven by elite athletes, slow and steady really does win the race. 


See the shoes elite athletes use for their aerobic training and find the option that’s right for you in our complete guide to support shoes


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