Being fairly new to the whole SUP thing but absolutely stoked on it I have found a few things out.
Ive paddled a Fanatic 14'0 with mates who paddle kayaks and its chalk and cheese. They kill me. But its a heap better sensation SUPing

I dislocated my knee and tore my ACL just before New Year in a windsurfing stack.
So to keep from going nuts I have taken to prone paddling my SUP to:
a: get out on the water
b: stop me from driving wife and daughter nuts.
Ive surfed from when I was a grom but my neck is killing me from holding it up(in a good way), I have the sorest nipples from continous contact with grip deck

(duct tape is your friend) and I cannot paddle continously for longer than I normally SUP. Totally agree with PTwoody, its bloody hard work but Im loving it as Im doing something on the water

Plus I stop every half a k or so and do my physio exercises which feel much better in the water.