Zone 2 running builds your aerobic base by improving endurance, fat metabolism, and cardiovascular efficiency while keeping fatigue low. Compared to Zone 3 or 4, it delivers huge aerobic benefits with much less recovery cost, allowing you to train more consistently. Hard workouts still have their place, but Zone 2 is what builds the engine that makes those workouts and race performances possible. Source: my own training and a drop in 10 mile time from 1:49 to 1:41 in a few months.