iOS: A good running mix can make your exercise more enjoyable, but building your own playlist takes work. If you want to skip that process, Cruise Control: Run automatically scans your library and builds running playlists based on the pace you want to run.
Cruise Control: Run has four ways to make a playlist. You can do a free run, where the app will try to match the rhythm of the music to your stride (you have to keep your phone in your hand); pick a target pace where it'll scan the BPM of your music library and build a playlist; set a heart rate goal that does the same thing with a heart rate monitor; or select a cadence and Cruise Control will adjust the tempo of songs to meet it.
You can make playlists to include just the songs you want, but if you opt to include your whole library you can vote down songs as they come up if you don't want to hear them again. As you'd expect, Cruise Control: Run also tracks your time, location, distance, calories consumed and speed. It's a little pricey at $5.49, but if you don't feel like manually creating your own playlists, it's a simple option.