The science of easy running: why most miles should be slow

Picture a 40-year-old runner who says her easy pace is 9:00 per mile. Her watch says her heart rate averages 158 bpm on those runs. Her estimated max is 180. That puts her at 88% of max on a run she thinks of as a jog, which is Zone 3 (threshold) in StartLane's model and a hard effort by most training models.

Most runners who have never worn a heart rate monitor are surprised by what they see the first week.

Easy running is the running you could keep up while holding a conversation, at roughly 60 to 75% of your maximum heart rate, and it should make up most of your training. Studies of elite endurance athletes point to about 80% of sessions easy and about 20% hard. In a Norwegian-style plan, around 70% of your training time sits in the easy zone, with most of the rest at threshold.

Where the 70% and 80% come from

The 80/20 idea comes from Stephen Seiler, an exercise physiologist who spent years studying how elite endurance athletes organize their training. Across many descriptive studies he found that athletes training 10 to 13 times a week tended to do about 80% of their sessions at low intensity, below roughly 2 mmol/L of blood lactate, and about 20% as high-intensity work. That is what people mean by "polarized" training.

Two details matter. Seiler counted sessions, not minutes. An easy run is long and a hard session is short, so by time the easy share is often higher than 80%. And the athletes in those studies were organizing themselves. Nobody assigned them this pattern, and they appear to have settled into it over years.

The 70% figure belongs to a related but different system. Norwegian-style training, as used in StartLane, spends about 70% of training time at low intensity and puts most of the remaining time at lactate threshold (Zone 3), with only a small slice at VO2max (Zone 4). That differs from the polarized 80/20 model, but both agree that most of the volume is easy.

There is also some experimental support. In a 2014 study by Thomas Stöggl and Billy Sperlich, 48 well-trained endurance athletes were assigned to one of four training approaches for nine weeks. The group using a polarized mix improved peak oxygen uptake by 11.7%, more than the groups built around threshold work, high volume, or interval training. The high-volume group, which trained mostly at low intensity, did not improve on those measures, so easy miles work alongside hard sessions and do not replace them. It was one study with a short time frame, so read it as supporting evidence that points the same direction as the observational data.

What slow miles do

Easy running looks like it is doing nothing. Inside the muscle, a lot is going on.

Sustained aerobic work increases the number and size of mitochondria, the structures that turn fuel and oxygen into energy. It also builds more capillaries around muscle fibers, which shortens the distance oxygen has to travel. Your muscles get better at burning fat at a given pace, which saves carbohydrate for harder efforts, and the heart gradually adapts to move more blood per beat.

These changes build over weeks and months of regular volume. They need time on your feet at an effort you can repeat tomorrow, and they do not need suffering.

Inigo San-Millan, a physiologist who works with professional cyclists, has popularized the term "Zone 2" for this kind of training. In his lactate-based model it is an intensity where lactate is made and cleared at about the same rate, and he argues it is especially good for building mitochondrial function. Not everyone agrees it is uniquely powerful, and "Zone 2" means different things in different systems. By his lactate numbers, about 1.5 to 2.0 mmol/L, it lines up with StartLane's Zone 2 (75 to 82% of max heart rate), the band just above the easy zone, which is one place where coaches disagree. The safe takeaway is that low-intensity volume builds the aerobic machinery, and you can do a lot of it.

Your tendons and bones also adapt to easy running, more slowly than your lungs and heart. A runner who builds fitness faster than their connective tissue can follow is the runner who gets shin splints. Easy miles keep the load manageable while the tissue catches up.

Why the middle is the problem

Between easy and hard sits a band of "comfortably uncomfortable" effort, roughly 75 to 82% of max heart rate, which is Zone 2 in StartLane's model. It feels productive. You finish sweaty and your watch shows a decent pace.

The cost is that moderate running leaves you tired the next day without giving you the specific adaptations of threshold or VO2max work. Run enough of it in a week and you arrive at your real workouts with heavy legs, so those sessions get worse too. Easy days have to be easy before hard days can be hard. We wrote a longer breakdown in Why You're Running Too Hard on Easy Days.

How to find your easy pace

Pace is a poor guide because it changes with heat, hills, wind, sleep, and fatigue. Heart rate is better, and a quick conversation test backs it up.

1. Estimate your maximum heart rate. The Tanaka formula is 208 minus 0.7 times your age. For a 40-year-old that is 208 - 28 = 180 bpm. A hard field test gives a better number but is not required to start.
2. Multiply by 0.60 and 0.75. For that same runner, 108 to 135 bpm is the easy zone. Her 158 bpm "jog" is 23 beats above the ceiling.
3. Run by that ceiling. If your heart rate climbs past 135 on a hill, walk until it falls. This feels silly for the first two weeks.
4. Check with the talk test. You should be able to speak in full sentences. If you can only get out a few words between breaths, you are above the easy zone regardless of what the watch says.

Expect the pace to be slower than you want. A runner who holds 9:00 per mile at 158 bpm might find that 135 bpm lands closer to 10:30 per mile at first. Over several weeks the pace at 135 improves, and that improvement is how you know the training is working.

Heart rate drifts upward during long runs and in the heat, so on a hot day or in the last miles of a 90-minute run, allow a few beats of drift rather than stopping to walk constantly. Caffeine, poor sleep, and illness raise heart rate too. A chest strap is more reliable than a wrist sensor for steady numbers.

What if easy feels too slow to count

Run-walk intervals are fine. Walking for a minute every ten keeps your heart rate in range, which beats running through the ceiling. You can also run trails or rolling routes by effort instead of pace, or run with someone slower than you and talk the whole way.

Track one number: your pace at a fixed easy heart rate, measured every two or three weeks on the same route. Seeing 10:45 turn into 10:20 at 135 bpm does more for motivation than any lecture on mitochondria. Hold the same heart rate for a couple of months and the pace that goes with it gets faster, though how much depends on where you start.

How much easy volume is enough

Take a runner with 5 hours a week of training time. Seventy percent of 300 minutes is 210 minutes, or three and a half hours, at easy effort. The remaining 90 minutes covers one or two quality sessions. If you only have 3 hours, you have about 125 minutes of easy running and a single quality day.

If you are new to running, there is no harm in spending your first few weeks on easy runs only.

How StartLane handles it

StartLane builds its zones from your maximum heart rate, using the Tanaka estimate from your age unless you enter your own number. Zone 1 is the easy zone, set at 60 to 75% of max, and it is the home of every easy run and long run in your plan. Zone 3 is threshold and Zone 4 is VO2max, so the hard work stays separate from the easy work.

When you set up a plan you enter your age, your current easy pace, your heart rate at that pace, and your weekly training hours. The Your Zones page then shows each zone's heart rate range and an estimated pace for it, based on the pace and heart rate you entered. Treat those paces as starting estimates and the heart rate range as the part to trust.

You can log runs by hand or pull them in with the Strava sync button or an Apple Health import, then compare your pace at easy heart rate from one block to the next. The daily check-in (readiness, sleep, soreness, HRV) can help you notice when easy runs are feeling harder than they should, which can be an early sign of fatigue.

For more on how the weekly mix fits together, read How to Run Faster Without Running More. If you want the zone math in more detail, see Heart Rate Zone Training: How to Calculate Your 5 Zones.