The theory of GET treatment (Graded Exercise Therapy) long advocated for ME/CFS ( Myalgic Encephalomyelitis/Chronic Fatigue) a disease similar to Long Covid, has at last been debunked through extended patient advocacy after it was found to be not only ineffective but dangerous, causing the disease to be aggravated, sometimes severely. In fact, for Long Covid as for ME/CFS, the most important treatment is REST. As several posters pointed out,  many such sufferers were highly athletic before being stricken with this condition.
This study is flawed and the article dangerous and should be pulled.
Just Google "CFS GET" to get the full story and extensive prior research.