I'm a runner. My music and playlists are on Google Music. I have all of my music downloaded to my device. I can't seem to figure out how to get apps like Runtastic or Nike+ to locate my Google Play Music. Any pointers? Do I need to manually move the files around on my device (android...Samsung GS3)?
I've had the same troubles. I had an iPhone for years and didn't even consider for a moment that accessing the music through your running app would even be remotely complicated as with iOS all all just works.
When I switched to Android I found that it's actually a really difficult ask. In fact the running app i have used for years, Map My Run doesn't even have a music option so I've begrudgingly gone back to Nike + (after trying several others in the process) as this allows me to access the music on my phone whilst retaining a half decent running app in the process (something that some of the others didn't quite manage to deliver).
I now use an app called Media Monkey, which is a bit like iTunes, although closer to Winamp, to move the music I want on my phone across to the device. It's annoying as I spent hours uploading all my music to Google Play and setting up my playlists only to discover that I can't access it through the Nike + app. I pretty much only ever listen to the music on my phone when I run so the day Google Play and Nike start talking to each other will be a very happy day indeed! :)