However there is another problem. If the body is very short of ATP, it can make a very small amount of ATP directly from glucose by converting it into lactic acid. This is exactly what many CFS sufferers do and indeed we know that CFS sufferers readily switch into anaerobic metabolism. However this results in two serious problems - lactic acid quickly builds up especially in muscles to cause pain, heaviness, aching and soreness ("lactic acid burn"), secondly no glucose is available in order to make D-ribose! So new ATP cannot be easily made when you are really run down. Recovery takes days!
When mitochondria function well, as the person rests following exertion, lactic acid is quickly converted back to glucose (via-pyruvate) and the lactic burn disappears. But this is an energy requiring process! Glucose to lactic acid produces two molecules of ATP for the body to use, but the reverse process requires six molecules of ATP. If there is no ATP available, and this is of course what happens as mitochondria fail, then the lactic acid may persist for many minutes, or indeed hours causing great pain. (for the biochemists, this reverse process takes place in the liver and is called the Cori cycle).