There is likely many a cause, it also being quite hard to fully describe symptoms as a cause.
Cause of the cellular dysfunction is a low-grade viral infection, caused by multiple viruses (i.e. whatever was dormant in your system since you were a kid). Cause of the viruses acting up is a lowered immune system caused by a physical, chemical, or emotional stress; i.e. It can be triggered by such things as an accident or surgery, chemo, Rx, antibiotics, job loss, or a divorce.
Cause of the fatigue is the inflammation and low ATP energy in the cells from the above.
It all snowballs, and is hard to get rid of except by eliminating ALL stressors (including any patented chemicals =Rx drugs, pesticide residues), taking anti-virals, supplementing the Kreb's cycle with magnesium, d-ribose, CoQ10, Omega3s, L-carnitine, and vitamins; and (most importantly) rebuilding the immune system through healthy 'clean' eating and living. It does not happen overnight!
Endurance can be rebuilt by ALWAYS keeping your heart rate under the Aerobic Threshold (AT), which is usually right around 60-65% of your Maximum Heart Rate, which is around <120 for the average 40+ person. This keeps you in aerobic 'fat-burning' mode, which is important. Going anaerobic (burning sugars) really screws up the energy pathway in CFS people and causes flu-like aches or a burning lactate build up.
Wear your HR monitor (with an alarm if it goes above your AT) while walking, doing yardwork, playing with the kids, etc.
Dont eat sugars and simple carbs.
Do eat more 'good' fats, including MCT oil, Extra Virgin Olive Oil, grass-fed butter, organic coconut Oil, and cold-water fish (Omega3 oil).
If there are hills on your route, get an electric bike or go *really* slow & keep the HR down, or find a new flatter and shorter route. If you can do the same pace and distance the next day, you are doing well, otherwise you are overdoing it. A study showed that CFS people can only do 60% as much of a maximum effort the next day, while Normal people (incl. couch potatoes) can do the same amount or more.
Most CFS people take an average of 49 hours to recover from an hour of exercise (normal people take 4.5 hrs) per one study, so you may only be able to ride every other or 3rd day at first.
It took me a year to ride 25-30 flat 12 mph miles (my normal retiree group ride) 2 days in a row. That is a good sign of progress! I progressed quickly after that.
Read 'Endurance Training and Racing' by Dr. Philip Maffetone to learn the benefits and why riding slow works. He mostly talks about training high-caliber athletes, but it holds true for CFS people also.