I Am convinced that the exercise intolerance is a symptom of something wrong like in my case It was the OI. All I know is that I had exercise intolerance, I have been trying EVERYTHING for exercise for a year (check my blog). Slow, laying down, and I crash over and over again. I treat OI (florinef) and 2 days later I could lift weights and exercise for a full hour with no PEM and I have kept going. I am back to work full time 40 hours. Go on family outings.....
If the EI was like a permanent condition or something else, is impossible that I had gotten better over night without having to build up. This just doesn't make sense that I can go on and not crash. I have not push over one hour because I am afraid to push my luck, but I feel I could keep going if I wanted to.
My theory: I think those who build up exercise are treating the OI (indirectly) and maybe that is why overtime they overcome and get better.