Many plans start with a base phase of mostly easy running or riding. The question is how long it really needs to be before structured intensity begins.
Base work builds capillary density, mitochondrial capacity and the durability of tendons and bones
For beginners, several months of mostly easy training before regular hard sessions is sensible
Experienced athletes often keep some intensity even in base phases, such as strides, hill sprints or short tempo efforts
Base length depends on history: returning from a long break usually needs a longer base than a short off-season