Earlier quoted context omitted.
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.
Dang! 45 minutes is not slow for a 10K (depending on your age and overall fitness, obviously). My pace for a half marathon is slower than that.