Experienced distance runner here. This is not new or mysterious.
> Overtraining Syndrome is where a lack of recovery from training stress results in underperformance, often combined with Mood Disturbances such as reduced Motivation, depression and anxiety
> Overtraining Syndrome is where a lack of recovery from training stress results in underperformance, often combined with Mood Disturbances such as reduced Motivation, depression and anxiety
https://fellrnr.com/wiki/Overtraining_Syndrome