Yes, it's a good workout.
Longer strokes feel like they have more power. But you do get plenty of power from short strokes too. Paddle is in the prime power zone for longer (on short strokes). Set yourself a course, and paddle at 55-60 stokes per minute, do it again at 40 strokes, and repeat a few more times.... observe results.
www.davidkalama.com/http://www.davidkalama.com/2010/12/14/use-your-hips/And get 2 paddles.