Wow, and I thought I was suited to long distance :-) I definitely tend to row longer -- from 5Ks to an hour -- but I can't say I don't feel it.

Sanity-checking: what pace are you setting? Or: what distance are you covering in 45 minutes? The Concept2 has almost no floor, so it's possible you almost aren't exerting yourself.

I can do 10k in 45 minutes but I wouldn't be able to sustain this all day. If I reduce the intensity by 20~30%, I could go indefinitely.