There is growing evidence that Long COVID and Chronic Fatigue Syndrome are either the same condition or very closely related.
In the last five years or so there has been a large amount of evidence that ME/CFS is an energy deficit disease. Body mechanisms aren't producing enough energy. One of the defining symptoms is post-exertional malaise (PEM) which is a fancy term for saying that where a normal person might recover from exercise in hours or days, ME/CFS folks will take days, weeks or months to recover.
There is currently no test for diagnosis, so it is largely a disease of exclusion of other fatigue conditions. This affects the health care system since a bevy of test are done (heart, lungs, blood) with no definitive results. At the same time, more extensive and often specialized tests reveal significant problems (2 day CPET, often more than 20% loss of blood to the brain, blood not being able to fill the heart prior to pumping, ineffective mitochondrial function, endothelial dysfunction)
Conventional and complementary therapies are ineffective. Think of being a castaway on an island where there is a lack of food and you are slowly starting. There are plenty of studies that show that letters home, meditation, massage, physiotherapy, supplements, etc will make you feel better, possibly for weeks or months, buy eventually they make things worse because they deplete your energy resources. To make matters worse, feeling better means doing more, which means overdoing it, which means depleting your energy even more. The common doctor strategy of "doubling the dose" creates even more problems.
Insurance companies have been relying on the cruel strategy of graded exercise therapy (same as for most physical rehab... slowing increasing the amount of exercise over weeks) to force people back to work. Refer to the paragraph above for the effect on the patient. This technique was based on a large flawed study. Current recommendations consider it ineffective and possibly harmful.
In Long COVID, people currently suffering from the condition have immediate onset. Surprisingly, they are often people who led very active physical lives before COVID. There is a sense that the number are only the tip of the iceberg. In the castaway analogy, you may function pretty well for years, depending many factors, but suffer a steady overall decline.
The only current "treatment" is "get plenty of rest and don't overdo it", called "pacing". For some people that can look like a reasonably active life. For others, going to the store for groceries or watching television for an hour can debilitating and require hours or days of bed rest. Note that your brain uses about 20% of your energy, so pretty much any activity such as reading, watching television, listening to music is just as bad as physical exercise. For many people, even standing or even sitting for short periods (like an hour) is problematic.
On the good news front. Long COVID has resulted in a sudden rush of funding. Most of the highly underfunded ME/CFS researchers (we're talking big league people such as Harvard, Stanford) have simply moved to adding the COVID keyword to their funding requests.
We've also started to see a recognition of the problem through the establishment of Long COVID treatment centers (one in Vancouver at St. Paul's), however most GP's are not attuned, particularly if you were relatively asymptomatic originally. Fatigue is such a common complaint that GP's will run the standard tests, then prescribe rest or worse yet, diagnose depression and start prescribing medications.
If no cure is found soon this problem will slowly and silently become the next legacy of the COVID crisis.