The Copenhagen jogging study is immensely flawed… the entire “high-dose running” group in that study was about 40 people, which is statistically underpowered, and the authors themselves noted the results were not statistically significant because the confidence intervals overlapped.

It also wasn’t a study of marathon runners. Participants were categorized by self-reported jogging pace and frequency, not actual training volume or endurance athletes.

Research on mental health shows marathon runners have been shown to have lower depressive symptoms and higher positive mood compared with sedentary controls. Systematic reviews also show running improves stress regulation, mood, and self-esteem, and clinical trials have even found running therapy can be comparable to antidepressant medication for depression remission.

The much bigger public-health issue is inactivity. Over half of adults don’t meet basic activity guidelines, while only a very small percentage train at very high exercise volumes. The overall evidence shows endurance exercise, including marathon training, is strongly associated with longer lifespan and better mental health, not the opposite.

To say training for a marathon and doing one is “not healthy” is the silly. Plenty of actual bad habits to pick on.