The Aerobic System: Your Endurance Engine

Why most of your running is easy

Easy running is the part of training you can repeat most days, and volume carried over months is the biggest lever most runners have. A study of the 16 weeks before 151,813 marathons run by 119,452 people found that the fastest runners trained more than three times the weekly volume of slower runners, mostly as easy running (zone 1 in the study's three-zone model, zone 2 in a five-zone model). The share of training in zone 1 rose in progressively faster groups, and time in zones 2 and 3 stayed relatively stable. It is observational Strava data, so it describes what faster runners do and cannot show that doing it made them faster¹.

So the extra running goes in at an easy effort, not at a moderate one. The simplest check that a run is easy is the talk test: you can speak comfortably. A 2022 review found that in laboratory studies the point where speech first gets difficult matches the ventilatory threshold². The guide on easy and recovery runs covers how easy is easy enough, and the guide on polarized training covers how the easy and hard running fit together.

What easy running builds

Easy running builds the machinery that turns fuel and oxygen into energy. A meta-analysis pooling 353 training studies found that muscle mitochondrial content rose 23% with low or moderate intensity continuous training, against 27% with high-intensity training and 27% with sprint intervals. Capillaries per muscle fiber rose 15%, 13% and 10%³. Continuous low to moderate effort, where easy running sits, did nearly as much as hard training for mitochondria and as much or more for capillaries.

Hard training raised mitochondrial content faster per hour of training, and training more often was associated with larger gains: six sessions a week beat four, which beat two³. Hard work gets more per hour, and easy work is the kind a runner can repeat most days, which is why the weekly total ends up easy. The three measures that set race speed also depend on what you add to it: see the guides on VO₂max, lactate threshold and running economy.

How long it takes

In the same meta-analysis, capillary growth came mainly in the first four weeks, in untrained to moderately trained people. Mitochondrial content kept improving for longer: sprint intervals were particularly effective early on, while easy and high-intensity training showed slower but steady improvements over a greater number of training weeks³. A first change in a few weeks is normal, and the rest builds over months of regular running. Add volume in small steps (see the guide on progressive overload).

People respond differently, and the same person does not respond the same way each time. In 42 untrained adults who completed the same 8-week program twice, each person's gains were poorly reproducible from one block to the next, and the authors concluded that adaptability is not a stable trait of the person⁴. The study was of untrained adults, not runners. Stay consistent, and do not measure yourself against someone else's timeline.

How to tell it is working

You do not need a lab to see the base building. Watch for these over weeks, not days:

• Your heart rate sits lower at the same easy pace, or your pace is faster at the same easy heart rate. Heat, a poor night's sleep and hills move heart rate too, so compare runs in similar conditions.
• Easy genuinely feels easy. If speaking comfortably takes effort, the pace is above the point the talk test marks as easy, as a 2022 review of it found².