Sounds like you have been running! Which in my mind makes you a runner!
Christopher and Eric talk about how fixing running form fixes multiple issues in our bodies, one of the top five being lower back pain. Obviously, I have no idea how bad your pain is and if running is safe for you or not, so I totally trust you'll keep doing whatever makes sense to you and please know there's stuff in their book specifically about lower back pain and running and form.
Running in -5'C!! I haven't done that. The closest I've got, I think, is running at 1'C with a wind chill factor that dropped it to -3'C for a parkrun in our Snowy Mountains here in Australia a few years ago. Cold, cold, cold!
RE: I've decided to do a marathon this year! (Did I really just say that?! 😅🤔😛)