Regarding C2 performance, I like to go by preset times rather than distance. I've been doing 3*20mins or 3*30mins, going steady on the first two and hard on the last one. I did under 1:50 for the final 20 last week (1:49.5), and sub-1:52 for the final 30 (1:51.8 - felt harder than the sub 1.50 20min tbh).
Regarding physique and times - the lighter rowers seem to generally be better at the distance runs, whereas the heavier rowers are better at the high-power sprints. When I was rowing I used to beat the rest of my boat easily over 500m / 1K, but some of the guys who were 10kg lighter than me would regularly kick my ass in the 5k / 30min ergs.
At the moment I can do 1:24 for 500m, but a bigger guy like morba could almost certainly go sub 1:20 without much effort.