Lucinda Bateman has written about something like this. Essentially, the idea is a CFS patient needs to stay below their personal anaerobic threshold. Where that is varies greatly across patients.
See
http://www.iacfsme.org/portals/0/pdf/cfsandtheexcerciseconundrum.pdf
"There is no doubt that my hundreds of patients who meet the CFS case definition exhibit a wide
spectrum of exercise tolerance" ...
"Staci Stevens, MS, an experienced CFS exercise physiologist in California, instructs her patients to
wear a heart rate monitor with an alarm to notify them when the heart rate has climbed to a
predetermined level. She measures a CFS patient’s anaerobic threshold objectively during graded
cardiopulmonary testing, notes their heart rate at the anaerobic threshold, and then uses that heart rate
value to estimate the anaerobic threshold during physical activity. It is typically somewhere between
90-110. (Linda’s was 80!) Staci counsels patients not to exceed that heart rate during physical activity."
I came to this understanding for myself, based on my own personal experience, before ever seeing anything Bateman had written. I eventually came across a statement about this in a powerpoint of Bateman's.
I can walk for fairly lengthy periods of time - I just can't get my heart rate up too much. If it seems I'm in a lower-energy CFS period (it seems to wax & wane somewhat with me over months, roughly from 65% -80% of what I feel like used to be normal), I can't walk as much - I'll get the PEM. I have to really pay attention to my body & not push it.
I may not completely avoid PEM, but it's fairly mild, and for me, the benefit that comes from walking, in the form of exercise & meditation, makes it worthwhile.
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About this statement in the OP: "Why do we get so many symptoms that are so intense and these symptoms are so disabling pretty much from the get go?"
This isn't true for everyone. Many with CFS have slow-onset. Most take a lot longer to get a diagnosis, because the change is so gradual. Thus, these people are undercounted. I'm one of the people who had a slow onset.
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My experience is similar to Wishful's (see a few posts above) - socializing, especially when it's with people other than close, trusted friends (but I have to limit it even with them), triggers PEM for me, too.
"Cerebrally-induced PEM, from driving or socializing, triggers my PEM after a much shorter delay (half an hour to a few hours)"
I can't shovel soil for hours as Wishful can. I can do things like that in small bursts, but end up resting between. If I push it too much, the PEM is more noticeable - I won't be able to do as much over the next few days.