Do You Really Need to Run Slowly to Improve?

“To run faster, you have to run slower.” You hear this everywhere. And for the past few years, polarized training – the famous 80/20 rule – has become a kind of absolute truth. But does it truly work for everyone? And is it as well-proven as people claim? I decided to look into it.

The 80/20 Model: Where Did It Come From?

The idea dates back to the 1990s. Norwegian researcher Stephen Seiler observed that elite endurance athletes consistently distributed their training: approximately 80% of their volume at low intensity and 20% at high intensity. The intermediate zone – the infamous “gray zone” – was rarely utilized.

Seiler found this pattern among cross-country skiers, rowers, and cyclists. And several studies have shown that this distribution yielded better results than approaches with more moderate-zone work. Matt Fitzgerald's book popularized the concept, and since then, it has become somewhat of a running mantra.

Why It Works

The Aerobic Base: The Foundation

The main argument is physiological. Easy runs develop aerobic capacity without excessive fatigue: the heart ejects more blood with each beat, capillaries multiply in the muscles, and the body learns to utilize fat more efficiently. All of this helps to increase VO₂max and push back the fatigue threshold.

Easy Running = Active Recovery

Running at an easy pace — 60 to 75% of your maximum heart rate — allows you to accumulate volume without exhausting your nervous system. This is what enables elite athletes to run 90 to 125 miles per week without breaking down.

Running Economy Improves with Repetition

The more you run, the better you run. Repeating the movement, even slowly, improves efficiency: less vertical bounce, better relaxation, a more economical stride. This isn't something you measure in a single session, but it becomes apparent over months.

Limitations of the Model

What Works for Elites Doesn't Necessarily Work for Us

Seiler's studies focus on athletes who train 15 to 25 hours per week. For an amateur runner with 4 to 6 hours available, it's a different story. If you dedicate 80% of your 5 hours to easy running, you're left with very little time for quality work — sometimes less than 45 minutes of high intensity per week.

Researchers (University of Stirling) have suggested that for moderate-volume runners, a more pyramidal distribution (more threshold work) could be equally effective. The debate is far from settled.

The Norwegian 'Double Threshold' Approach

Since 2020, the Ingebrigtsen brothers' method has shaken up long-held beliefs. Their approach: two daily lactate threshold sessions — precisely the intensity the 80/20 model typically avoids. The results are spectacular: world records in the 1500m and 5000m for Jakob.

Should we ditch 80/20? Not necessarily. The Norwegian approach is for young, highly supervised athletes who train twice a day. Applying it to a 42-year-old runner preparing for a half-marathon is risky.

The Real Problem: We Rarely Run Slowly Enough

The advice “run slowly” is easy to give, but hard to follow. Personally, it took me a long time to accept running at 10:28/mile (6:30/km) when I knew I could run 8:03/mile (5:00/km). Many runners think they're going easy when they're actually in the gray zone — too fast to recover, too slow to create a true stimulus. The result: fatigue without progress. Checking your pace with a pace calculator based on your MAS really helps to objectify.

So, What's the Verdict?

Yes, most of the time, but not blindly. Here's what emerges from studies and practical experience:

How to Apply These Principles in Practice

1. Define your truly easy pace — use a pace calculator to set a realistic range, not just “what feels slow”
2. When it's hard, make it hard — lukewarm intervals don't achieve much. When you're scheduled to run fast, really go for it.
3. Don't neglect threshold running — for those running 4-5 hours per week, a weekly threshold session (half-marathon pace) can usefully complement the 80/20 approach.
4. Track your progress — comparing your estimated race times over the weeks helps verify that it's working.
5. Be patient — the benefits of slow running are slow to appear (yes, it's ironic). It often takes 8 to 12 weeks to see improvement.