I am experimenting with this at the moment, and it's surprisingly a lot of fun and you can get some decent speeds, on a decent longboard; and not to mention a great upper body workout.
Been doing it for a few months and aspalt is hard on the shoulder joints - I think if you're sub 30s then sure but after that it starts getting tough, unlike SUP'ing where you get a great workout without easily hurting joints due to the 'give' you get from water paddling vs hard street paddling. Having said that I like just longboarding on a big board - I have the Kahuna 59' Bombora with the bamboo big stick paddle. I do miss the paddle when I need to do a quick stop. 
