If you are listening to music then ducking makes sense. What if you want to listen to a podcast or an audio book while you are training? Wouldn't pausing the track make more sense, and resuming it when the audio cues stop?
Either you'd need to use an Android intent to make the player stop and restart (I mean the players that users select by touching the music icon (two quavers) top right in the Android app), -or- you would need to add your own Runkeeper music-playing service. Probably you don't want to do that, so if none of the currently supported players support it, look for one that does and support it.
Settings screen could then have an option "prefer pause over ducking" that would pause if possible but revert to audio ducking otherwise.