Auntie C,
I was using my phone until I came up with this brilliant idea:
I made a 40 minute playlist, imported it into Garage Band, then inserted my voice-over saying "Run" and "walk" at the appropriate times. That way I only had to carry my ipod, and not worry about timing!
Then I can just change the playlist and leave the voice-overs where they are, or change their placement as I progress through the plan.
I just didn't like timing it because I would constantly be checking the time and 2 minutes felt like forever. And I don't own a programmable stopwatch.