Rowing didn't work for me because I have a thin bony ass. The hardness of a rowing machine seat would do me in after 20 minutes.

Great workout for 10-20 minutes. How do people over come the bony ass syndrome on those hard seats?

As a long-time rower prior to switching to triathlon, a few comments:

1) I would agree that as a low-impact fully-body workout rowing could be a good option for working more on your aerobic base if you are getting enough running in already.
2) But, you need to use the right technique in order to avoid injuring yourself. You can find videos on the web (eg on the C2 website https://www.concept2.com/...ing/technique-videos) but if you want to do it regularly I would strongly advise trying to find a real rower who can give you a few short coached sessions to get started.
3) In terms of sessions, if you want to work on aerobic base only then you don't neet to do anything particularly complicated:
- Use an HRM to make sure you are in the right zone, and try to build up the sessions to something like 3x6km with a couple of minutes rest to drink and briefly stretch in between at a stroke rate of around 20. At the beginning that is likely to be too much so I would say start with perhaps 3x3km and build up from there.
- The "drag" setting on the machine should not be too high (something like 4 out 10 on the C2 or around 130 if you go into the menus and get the actual reading.)
- Like on a static bike, having a fan will help keep you cool.

I am currently using a rowing machine as a substitute for swimming as I do not have access to a pool. I have always used a resistance of 9. I thought that I would get more "value" out of a session with more resistance. I will try a lower resistance and see how I go.
Thanks for the tips.