My PEM hits 3-4 days later. I feel find until then... not that I'm extremely overdoing. I couldn't say what would happen if I ran a marathon.

But for 'normal' overdoing -- moving house, walking through large airports -- I do fine in the moment, although I'm not as strong as I used to be. I might get a bit out of breath or a little tired, but I don't get PEM symptoms.
If my OI is not under good control, however, I can have immediate OI symptoms from overdoing and/or being upright too long. Unlike PEM symptoms, my OI symptoms from overdoing can be alleviated by rest within about 24 hrs.
So for me and my daughter at least, there appears to be two parts: exercise intolerance from OI (when it's not under good control) which happens immediately and is relieved by rest within 24 hrs, and PEM (which has different symptoms) and shows up days later and lasts much, much longer.
I would not say it's an adrenaline situation that keeps me going during overdoing. For me, it appears to be a simple matter of whatever the phenomenon is (cytokines? inflammation? metabolic dysfunction?), it takes a few days to build up to the point where the symptoms are noticeable. The symptoms then increase for another 3 days or so and then fade very gradually over 2-8 weeks.