I just change the length of songs on iTunes, so that the song changes when I'm supposed to go from jogging to walking or vice versa. All you have to do is put the music you want in a playlist, then right click on each song and select "Get Info." Under the "Options" tab, you'll see "Start time" and "Stop time" and you can adjust accordingly. The track length for the song on the playlist won't look like it changed, but it has. Just remember to change it back if you actually want to listen to the whole song.
This is also good for songs that may have a slow start- you can just skip to the faster part.
Oh, that's interesting. For a minute I thought you were stretching or scrunching the track - going from Darth Vader sings 'Brown-Eyed Girl' to the Chipmunks singing 'It's My Party'.
I didn't know iTunes had that function. Not surprising since I don't have an iPod. I've messed with iTunes mainly for self-education, and that my music was 'gettable'.