OK,

Rithínn == I used to run.
Bhínn ag rith == I used to be running.

I assume that's correct? But when would you use one over the other? Can anyone give sample sentences in English where one would be preferable over the other?

I don't know if there's a huge semantic difference. The best I could come up with was this:

I used to run (rithinn) on Sunday afternoons,
so I used to be running (bhínn ag rith) when the match would be on the tele.