I haven't tried land paddling, but I know for it to mimic SUP closely you need a specific board. If you use a normal longboard you'll be forced to stay in "surf stance". If you get one that's super wide you can paddle with your feet side-by-side like you would on a SUP.
The other thing that I'm not sure about, is the difference in the paddle strokes. With water SUP the paddle goes into the water. With land SUP the paddle cannot go into the ground, so I'd expect it to behave differently.
A lot of people make their own land paddles for cheap. I'd say make a paddle, borrow a longboard, and give it a shot!