Hi fitzy & welcome to our crazy club!
I can't run and walk much anymore, but can still cycle and exercise quite a bit, often only in short stints. And I know this phenomenon from many different situations: When you let the tension off and the cortisol & adrenaline start to decrease again some kinds of hurt starts - overall Ache and local pains. That means that the fight / flight hormones are acting sort of anaesthetic, which is brilliant for emergencies, but not for mid- or long term.
Running & walking would also do this for me, I would be able to, the quicker the less pain then, the more pain after. So it's helping us going over our limits, ignoring the sweet spots, making us overdo it and not only increasing mid-term pain, but also wearing & tearing at our generally energy "spoons". As my fibromyalgia was probably partly caused by exactly this, not stopping in time, my suggestion would be that you aren't doing your body a favour. But I know it's hard to let go of a part of our old lives and adopt new habits.
It's possible that if you don't, that you will crash completely and will no longer be able to do anything in this direction any more. And it's possible if you do for a time and learn to find your sweet spots, that you will be able to enjoy that and add other routines that are more apt for your momentary condition. You might like to try finding your sweet spots where it no longer happens. I'm sometimes able to cycle >20mph for a few minutes (sometimes also for 10-15), but I now know how my legs feel when I need to regulate down again and how much.
