From the Coach guide
The run that builds your base, and most of your week.
Easy runs build your aerobic base. Your heart, blood and muscles adapt to more running, with little stress.
Run at a pace where you can talk in full sentences. Stay in zones 1–2. Slow down on hills and in heat.
Most runners do about 80% of their sessions easy. This lets you run more often and be fresh for the hard days.
Recovery run: shorter and slower, the day after a hard run. Long run: the same easy effort, for much longer.
Review of how endurance athletes train
In studies of competitive endurance athletes, about 80% of sessions were at low intensity and about 20% included hard work.
The review describes what successful athletes do. It does not prove that one exact split is best for every runner.
9-week randomized trial; 48 well-trained athletes
Training that was mostly easy with some hard sessions improved key endurance measures more than threshold-heavy, interval-heavy or high-volume training.
The athletes were already well trained, and the trial lasted 9 weeks.
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