When you select running mode, you need to tell the app the initial pace to start the selection process from (probably because I was standing still rather than running when I launched the app). I started at 160 bpm as they recommend and then they asked me to select a playlist of songs from which they would choose music for me. I chose the epic playlist and it seemed to pick suitable songs whose beat encouraged me to stay going at the same pace (i.e. they weren't racing along like a Darude song which would encourage me to sprint and at the same time they didn't pick relaxing Whale sounds which would have slowed me to a walk).
Overall, I think that I like this feature and will use it again. In fact I ran 10km with this run rather than my more normal 5km route so this is proof of its effectiveness. One complaint I would have is the fact that it kept repeating the same song - maybe I was stuck at the same pace but still it could get boring on a long run.
One interesting thing I noticed is that Strava interacts with Spotify by turning down the music when it has statistics to report, so it is easier to hear. In contrast, BeyondPod pauses the podcast playback completely until Strava has finished reading out stats.