I know if you have an Ipod Touch there is an app you can use.  (http://c25kapp.com/), but that will not work on other ipods.

Once thing you can do is right click on a song, click "Get Info", then click on options, and change the stop time to when you want the intervel to end.  (The song must be longer than the intervel, or you can have 2 or more songs, and time the last one right).  If you do it that way, when the song stops, you switch it up.  Thats what I do, and I find it very easy to use, plus you can have a fast paced song for the running, and a slower paced song for the walking.

http://www.sbellcheck.co.uk/couchto5k/Couch_to_5K/Podcast/Podcast.html