I am a useless SUP'er but I try. Try irregularly, that is...
It is good rehab for all my windsurfing injuries.
Anyway, as an idea of skill level I am a bit overweight at 100kg

in off season, and have a 10' x 33.5 " JP widebody. I can paddle around all day in 6-8kn and small swell and not fall off. Riding waves I suck, but have caught a few.
Now:
When I started I got sore arches under my feet and then saw tips along the lines of don't clench your toes etc, just relax. I thought yeah that make sense I can feel what I am doign with my toes all clenched up, so I relaxed my feet and it is all good.
Now after not SUPing for a few months, back out on the water and I am fine for 15mins and then feet get a tiny bit numb. Could stay out for ages, but it is there... Seems not to be from tensed up feet.
So after I get off the water it is worse, pins n needles in my feet like when you fall asleep on your arm and if you poke it, it feels dead. Feet not quite so bad, but enough.
I also have discomfort in my sacral area (spine meets pelvis).
This all goes away after 10mins packing up and then driving home (2mins)
Given I have fkd up hip, broken left ankle and suspect knees from windsurfing, and previous (minor) back issues, I wonder if it is not technique when SUPing and perhaps it is a nerve pinch.
On the other hand - it does not last long, so is it normal for unfit useless bastard beginner SUPers?
Anyone...?