I've been looking at making a 30 minute mix tape for my walk-run workout.
So, really all you need are some MP3s of your favorite songs, and an audio tool to put it all together.
First, get your MP3s together.
Get an multi-track audio editor. There are many free ones:
Acid Xpress http://www.acidplanet.com/downloads/xpress/
JoKosher http://www.jokosher.org/download/
TrackAx http://bit.ly/MhZhG
Audacity http://audacity.sourceforge.net/
Once you are familiar with the program, drop in your MP3s in the order you want to hear them. Here is an excellent step-by-step using Audacity. Drag them under each other for some cross-fade action.
Some will do this for you.Export the whole thing as an MP3 and put it on your iPod/MP3 player.
If you are running the C25K, get the Audio Cues on MP3 (click the MP3 link at the top) and time your music to the voice-overs.