Some friends and I have recently gotten SUP we paddle on a river sometimes calm and sometimes lots of waves so it’s a good variety. None of us have been very sore we get tired but not sore so I am wondering if we are doing something wrong?
Maybe you are already fit
If it is not making you sore then you are not taxing your muscles so maybe you already have good balance and are already fit.
Try it in some waves or try paddling faster if you want more of a work out.