Rockmyrun.com, and the mixes are designed by popular dj's.
1 free credit on registration
When you register, you automatically get '1 credit'. You can use credits to download a mix, and 1 credit gets you a mix of 30 minutes. If you want longer mixes, you need more credits. You can buy them, or you can save up. Every month you get one free credit.
I downloaded the mix 'Golden Goodness', because it had lots of 'oldies' in it. Unfortunately it wasn't nearly enough Country for me, so after about 5 minutes of Golden Badness, I quickly switched back to my familiar songs about abandones orphans and wives.
But hey, there may be a music mix for you on rockmyrun!
So if you're interested in getting a free musix mix for your next run, run on over to Rockmyrun.com!