Not just for certain types of lethargy, I'd say - it also works with certain types of exercise. Namely, regular, not overly strenuous exercise will help build up more energy over time.

Suddenly climbing mountains does not fall into this category, and even people who regularly exercise can have an off day or week.

Too true on people not accepting some physical challenges. "No, really, I can't do that, I've been finding my limitations for a quarter century" does not stop people from offering clever ideas on how I just might make x work.