How it works
You spend most of your training time running, riding, or swimming easy, below your first threshold (roughly Zone 2). At this intensity the body responds by growing mitochondria (the cells' aerobic power plants), adding capillaries to deliver more oxygen, and improving fat oxidation so you spare glycogen. These adaptations raise the speed and power you can hold aerobically, which is the ceiling most endurance results depend on.
The Lydiard tradition
New Zealand coach Arthur Lydiard popularised periodising a season as base first, then sharpening. Athletes log a long block of high aerobic mileage to build the engine before layering on hills, tempo, and speed in later phases. The logic: speed work is far more effective, and far safer, when it sits on top of a deep aerobic base, so the base phase is treated as the non-negotiable foundation rather than filler.
How to apply it
Keep your easy days genuinely easy: the most common mistake is letting Zone 2 drift into Zone 3, which blunts the adaptation and adds fatigue. Anchor intensity to heart rate (a Zone 2 cap or the MAF method from Phil Maffetone) rather than feel, and build weekly volume gradually so tissues adapt without injury. Be honest with yourself that the payoff is slow: aerobic gains accrue over months, and the discipline to hold back is the whole point.
FAQ
How long does it take to build an aerobic base?
Think in months and seasons, not weeks. Mitochondrial and capillary adaptations accumulate slowly with consistent easy volume, so a meaningful base typically takes several months, and the deepest bases are built over years of steady training.
Why do I have to go so slow?
Easy aerobic intensity is what triggers the specific adaptations (fat oxidation, capillarisation, mitochondrial density) that raise your aerobic ceiling. Going harder shifts the stimulus and adds fatigue without improving the base, which is why disciplined slow running is the core skill of base training.
How is aerobic base training related to MAF and Zone 2?
They overlap heavily. Zone 2 describes the easy aerobic intensity band, and Phil Maffetone's MAF method is one practical way to set a heart-rate cap (around 180 minus your age) so you stay in that band. Both are tools for keeping base work easy enough to do its job.