Ask what the plan is building
A base period might emphasize tolerable weekly activity, longer comfortable sessions, or familiarity with a sport. Those are different goals. The phrase becomes more useful when the plan names its intended outcome.
For someone new to exercise, reliably finding time and a suitable activity may matter more immediately than dividing training into elaborate phases.
Easy does not mean the aerobic system is otherwise absent
Aerobic energy production contributes across a wide range of intensities. Harder exercise does not simply switch it off. Likewise, an easy session is not proof that it uniquely targets every adaptation described as “aerobic.”
Training effects depend on the dose and starting point as well as intensity.
A concrete version of the goal
Instead of “build my base,” someone might track whether a familiar route feels more manageable, whether they can complete their chosen weekly sessions, or whether a longer outing fits comfortably into recovery and daily life. These observations are useful without pretending to measure mitochondria directly.
There is no universal point at which a base becomes complete. A plan can gradually change as capacity and preferences change. For health-oriented activity, an elaborate base phase is not a prerequisite for benefiting from movement that already fits your circumstances.
