I'm not sure why exactly, but I would say "Mi devus ... " - "I had better go" - "Mi devus iri". It feels like it conveys the same idea to me.
Often there isn't a lot of need to distinguish. Something is in the past and that's that, but you can add more meaning if you feel you need to, for emphasis for example.
Mi faris - I did
Mi jam faris - I already did, but it feels more like "I have already done" in conversation
Mi estis faranta (or Mi farantis) - I was doing
Are you going to the Esperanto Congress in Edinburgh in May?