If you’re relatively new to endurance training, you might have heard of polarized training, but you might not know exactly what it entails. Most athletes new to the world of endurance training are far more familiar with a high-intensity model of training, popularized by programs like Peloton and CrossFit. It relies on frequent, short and hard, interval sessions. It’s the “no pain, no gain” approach to training.
Yet polarized training, based on how the best endurance athletes tend to train, is fundamentally different. [1–3] Athletes spend only 10-20% of their time going hard and 80-90% of their time training slowly. And when we say slow, we mean slow!
For new endurance athletes, particularly those coming from a program where they’re used to going hard a lot, it’s actually the slow training, not the interval work, that tends to be the hardest part. With limited time to train, going slow is neither fun nor does it feel beneficial.
But, while it may feel that way, it’s the slow work that produces the biggest gains in our aerobic systems—something that’s pretty critical to endurance sports. 
The importance of the aerobic system for athletes new to polarized training
Several years ago, a Tour de France rider showed up to our local masters training race. Everyone assumed he’d drop us all on the steep one-minute climb in the middle of our loop, but surprisingly he didn’t, and he wasn’t holding back.
His Tour cred showed up after the climb. While the masters had to soft pedal to recover, the Tour athlete rode away at 300 watts. Even more impressive, while few of our masters were strong enough to sustain 300 watts for more than 20 minutes he could hold that power for hours. Our masters could match a Tour rider for a short high-intensity effort, but they were beyond their limits just going his steady pace.
This was one of the best examples I’ve seen that it’s the slow sustainable efforts, not short high-intensity, that mostly distinguish elite endurance athletes. There’s a simple reason for this.
We all learned in gym class that we have two major energy systems: aerobic and anaerobic. The anaerobic system produces a lot of energy fast, but it’s short lived. It’s what we rely on when sprinting or lifting a heavy weight. The aerobic system is not as strong, but it effectively lasts forever. It’s what you rely on if you’re running a marathon or an Ironman.
Unfortunately, what we’re often taught in gym class is that we are completely aerobic until a certain intensity where we “go anaerobic” and switch energy systems. In reality, we are always using a mix of both systems. All that changes is the balance between the two.
New endurance athletes and athletes who have been following a high-intensity model tend to have a strong anaerobic system and a relatively weak aerobic system. This is why they can be surprisingly competitive with a Tour athlete for 30 or 40 seconds.
The problem is that while an elite athlete can go surprisingly fast relying almost exclusively on aerobic metabolism, the new athlete relies a fair amount on the anaerobic side of the balance at even moderate intensities. And since anaerobic metabolism doesn’t last long, the novice lasts minutes at a pace an elite athlete can hold for hours.
Commit the time to build your aerobic engine
Improving your anaerobic engine can take as little as four to six weeks. Building a strong aerobic engine is measured in years. If you are new, expect it to take you upwards of four or five years to build a solid aerobic engine. That may sound depressing but think of it this way: No matter how much stronger you get next year, you’re going to be even stronger the next year.
I experienced this with a 50-year-old master’s athlete who hired me to help him at his local Saturday group ride. He kept getting popped on the short climbs, so he wanted to work on his anaerobic power. But when I tested him, it was his aerobic engine that was weak. So I put him on a polarized program with lots of long slow riding.
We saw only modest gains the first two years. The third year he was not only getting over the climbs but attacking. The next year, he and one of the most respected category 1 riders in the province dropped the whole field and battled it out to the end.
We never improved his one-minute power. He was getting over the climbs because the race pace had become easier than his “long slow” training rides and he was hitting the climbs fresh.
We need to isolate the aerobic system to train it
It is true that all training builds the aerobic system , but as long as an athlete has an over-developed anaerobic system and underdeveloped aerobic system, the benefits of high-intensity work is limited. 
Early in my cycling career, I spent years following a high-intensity training model and kept losing in even the lowest level amateur races. Finally, I changed my approach and spent an entire base season doing almost exclusively slow riding for three months, riding at 14 to 15 mph on flat roads. That summer, I finished 12th in the Elite Nationals time trial.
To build the aerobic system, it’s necessary to isolate it as much as possible. Since most new athletes will rely heavily on anaerobic metabolism at even moderate intensities, isolating the aerobic system means training very slowly. This can mean running 12-minute miles and walking every climb, but even elite runners do that.
When doing your slow steady workouts (or 80-90% of your training) I recommend athletes pace themselves by heart rate. Target no higher than 65% of maximum heart rate or 75% of your threshold heart rate. So if your maximum heart rate is 190 BPM and your threshold heart rate is around 170 BPM, you should be training around 120 to 123 BPM.
Yes, that’s right, it’s going to be very slow—but the gains will be great.
 Seiler S, Tonnessen E. Intervals, Thresholds, and Long Slow Distance: the Role of Intensity and Duration in Endurance Training. SPORTSCIENCE 2009.
 Seiler KS, Kjerland GØ. Quantifying training intensity distribution in elite endurance athletes: is there evidence for an “optimal” distribution? Scand J Med Sci Spor 2006;16:49–56. https://doi.org/10.1111/j.1600-0838.2004.00418.x.
 Seiler S. What is Best Practice for Training Intensity and Duration Distribution in Endurance Athletes? Int J Sport Physiol 2010;5:276–91. https://doi.org/10.1123/ijspp.5.3.276.
 Laursen PB. Training for intense exercise performance: high‐intensity or high‐volume training? Scand J Med Sci Spor 2010;20:1–10. https://doi.org/10.1111/j.1600-0838.2010.01184.x.
 Coffey VG, Hawley JA. The Molecular Bases of Training Adaptation. Sports Med 2007;37:737–63. https://doi.org/10.2165/00007256-200737090-00001.