Overtraining happens right after you start out, because you're not used to the training at that point. You're suddenly running a _huge_ ATP deficit, and ATP (and other metabolites) can't be produced on a whim, metabolism has to adapt. Many people stop right after they started because they exhausted themselves too quickly, instead of waiting 2 days and continuing then. Everybody should be worried about overtraining. Not only for mental health, but also metabolic and heart health. You know, sudden cardiac arrest and things like that.
I don't disagree at all with doing regular training. I'm doing 10km walks almost every day now. But everybody's metabolic capabilities are different, and rest periods are highly individual. Rest isn't optional, ask any bodybuilder.
