I originally set up Itunes as my music for Couch to 5K and RunKeeper. But I have been building my music on Spotify and would prefer Spotify as the provider of music for these apps? Is it possible to make the switch?
I'm a big fan of Spotify and currently have it on my computer, but if I want to use it on my iPhone to listen as I run, it only allows me to listen to radio stations created out of my songs (like Pandora). I believe that if you want to have Spotify's "play on demand feature" on more than one device, you need to pay their membership fee (something like $5 per month). It's totally do-able, so long as you pay their monthly membership price.
Of course, when I look at the date that this question was posted, I suspect that you may have already found the answer to your question long ago, but hopefully if my answer doesn't help you, it can help some other community member : )