I think the whole notion of Post Exertional Malaise (PEM) might affect those of us who have been sicker longer. In my experience PEM is the result of the release of toxic substances that are stored in the body tissues after a tired system has been unable to properly clear these toxins out. Everyone, sick or not, must clear toxins out of their body. It's just part of life. When you exercise, these stored toxins get flushed out of you muscles and such, into you general circulation, where after some time, they exert their deleterious affects, and make you feel even sicker and more fatigued than you felt before. Again, I think think PEM might be more of an issue after you have had CFS for some time, allowing toxins to accumulate.