I used to row in high school, burned out and just this past semester got back into it for PE credit. The benchmark is the 2k, which from your 500m pieces you can get a decent understanding of.
I don't know if you've rowed before, but form is incredibly important not only for your wellbeing but also so that people that used to row don't look at you and laugh to themselves. (they do and they will)
I did my 2k for the end of the semester the other day and it was around 7:40, which isn't awful, but it's nowhere near where I was in high school, especially considering I weighed 15 lbs less then, as scrawny as I was.
I find 10k or 30 minute pieces to be quite nice actually. For you, you'd probably keep a split around 2:05-2:15 for the whole piece. How much did you rest between 500m pieces?