Training volume versus intensity
Evidence: strong
Total volume, mostly easy, is the strongest single driver of distance performance. Intensity is the finishing stimulus, not the foundation. What is less certain is the precise distribution of the hard part.
Across the available evidence, total training volume, most of it easy, is the variable most consistently associated with distance-running performance. In one study of 85 runners, total weekly distance was strongly correlated with performance, and easy running and tempo running together explained much of the variance in a regression model (Casado et al. 2021). That is a single cross-sectional study, so the exact coefficient should not be read as a fixed quantity; it is the direction that recurs. Elite distance runners typically cover 120 to 180 km per week, the great majority of it at low intensity (Tjelta 2016).
Volume and intensity drive different adaptations. One proposed reading is that volume raises mitochondrial content while intensity raises mitochondrial function, and that volume-driven gains reverse quickly when volume drops (Granata et al. 2018). The two appear complementary rather than substitutes.
The practical synthesis is consistent across observational and mechanistic evidence: a large base of mostly easy running is the dominant driver, and a small dose of well-targeted intensity supplies the finishing stimulus. Causal trial evidence in elite runners is thin, because randomising elite athletes to years of different training is impractical, so reverse causation cannot be fully excluded: faster runners may simply tolerate more volume.
This is the substance behind the “most training should be easy” message that the 20 discussion popularised. The defensible claim is the volume claim; the precise distribution of the hard part is more contested.
In practice
How to apply it
Build volume to the most you can absorb without breaking down, almost all of it easy, then add one or two quality sessions a week, no more. When time is short, protect easy volume first and trim intensity, not the other way round: the base is what most of the benefit rests on. Raise volume gradually and treat persistent niggles as the signal to hold or back off, since the ceiling on volume is tissue tolerance, not fitness.
The trap for the time-crunched
Runners short on time often do the opposite of what the evidence suggests, making most runs moderately hard to “get more in”. That moderate-intensity rut adds fatigue without the benefits of either truly easy or truly hard work. If you can only run a few times a week, keep the easy runs genuinely easy and make the hard ones genuinely hard.