Slower recovery is more important on the water. Newtonian physics, apparently - large mass of rower moves in opposite direction from small mass of boat, boat speed increases at first (reaction) then slows as weight pushes boat backwards through feet.
Not as important on a Concept 2, although if rowing at a high rating (high strokes per minute, spm) you can drive the machine across the room depending on the flooring and if you thump into frontstops. Also, the recovery (coming forward up the slide) is your opportunity to rest & relax. And yes, you need to, and can do, at 36+spm.
Technique is important; doing it right is more efficient and stops you f***ing up your lower back.
Rather than droning up and down, for interest, try:
3 x 10 minute pyramids - 4' at 20spm / 3' at 22 / 2' at 24 / 1' at 26 , then 22/24/26/28, then 24/26/28/30, with varying power ("pressure"), i.e. 4' light, 3min 1/2 race pace, 2' 3/4 race pace, 1' eyeballs out.
The first will feel quite easy (if you're properly warmed up), it's more difficult to force yourself to do the last one (or do another one...