I may be late on this but I came across a nice little script that sets up an interval training playlist that alerted me every 1 minute then 2 minutes and so on...
If you don't want to goof with the script you can do it yourself in iTunes, right-click on a song and choose - Get Info. In the Options tab, set the song's stop time.
Man, this made today's run go so much better. I was able to push out two extra sprints. Thought this might help a few others out there.