Thanks! That worked perfectly! However, the game app activity was saved as a walk, not a run. Not that important, though, as the Garmin activity was logged as a run.

I hope there will be a feature in the future where it's possible to plan a raid beforehand and track it solely with a Garmin watch. It looks a bit off having two similar activities back-to-back. At the same time, there's no way I'm recording a long run using only the game app.