- Moderately hard running is not “bad”, but it should not dominate by accident.
- Easy days, threshold work, long runs and faster work solve different problems.
- Progress often improves when training becomes more contrasted and more purposeful.
The common pattern: every run feels roughly the same
Many runners settle into a familiar pace: not truly easy, not controlled threshold work, and not hard enough to be a clear high-intensity stimulus. It feels productive because every run requires effort. But the week gradually loses contrast.
Why “always moderately hard” can hold you back
If easy days are too demanding, they create more fatigue than necessary. If quality days are not clearly targeted, they may not provide enough time at the intended physiological intensity. The result can be a lot of work without a clear progression signal.
What the research suggests
Systematic reviews of middle- and long-distance runners generally show successful training models with a high proportion of low-intensity work and smaller, deliberate doses of threshold or high-intensity work. That does not prove one universal 80/20 formula, but it supports the value of clearly different session roles.
PACE separates session roles instead of prescribing one generic “Zone 2”.
Easy, recovery, long runs and quality sessions have different purposes. The engine considers training phase, race date, real training load and recovery context, then distributes those roles without forcing a rigid percentage rule.
What to take into your half-marathon training
You do not need to make every run easier. You need each run to have a reason. A genuinely easy run should leave room for adaptation and future quality. A threshold workout should be specific enough to create the intended stimulus. A long run should build durability. As race day approaches, the mix should become more half-marathon specific.