For me at least, my underlying level of energy impairment is probably more important. I don't developed additional exercise intolerance (above and beyond whatever is normal when I start the activity) until the underlying impairment changes. Typically, I can do low-level activity pretty consistently for several years without getting better or worse at it, and then something shifts and I have to stop. The change is often prompted by getting a virus, after which I don't return to the level of function I had previously. It can last anything from six months to eight years, after which I improve again slowly.