If someone is deconditioned (not ME related) eg injury they can usually regain fitness progressively and fairly easily. The problem we have is attempting to do more can end up making us less fit.
This last month or so I've been more boom and busty I think from trying to increase activity a little. My POTS doctor thinks this is the next stage (but bear in mind he asked me if I was going to the gym when I was sitting in front of him in a wheelchair). I find if I do more I then flare up and I'm horizontal for a few days, bad for POTS deconditioning. This isn't what I'm doing, but if you measured in steps perhaps it would look like this:
300 steps a day comfortable = 2100 steps a week
600 steps pushed 1 day, 3 days bed 100 steps, 3 days 200 steps = 1500 steps a week
So staying in my limit keeps me fitter and is a better quality of life!
However as I think
@alex3619 said, this is a topic with loads of caveats. We may well be in different subgroups and my experience may not chime with you. For the POTS/ME subgroup I think it's also important to interpret PEM/fatigue correctly. Before also getting the extra POTS diagnosis I kept raising the severe difficulties I had walking (talking inability to get across the room sometimes). Medical staff told me that this was fatigue. Whenever I got concrete legs I stayed in bed because it was 'fatigue'. It's not fatigue, it's blood pooling and can be improved by salt, cardio drugs and compression tights. It isn't improved by resting in bed (although sometimes with POTS it is so exhausting you need to lie down). I deconditioned more unnecessarily.
In the past I got back to 95%, which was slightly mysterious but I didn't push myself I just gradually did more as I felt able. I probably don't need to say, but I didn't get worse again through deconditioning, there were definite viral triggers and probably over doing activity.
When I improve again I'm going to try hard to maintain fitness without over doing it.