Mine is a free app called Get Running, which I found through a link from the Runners World website.
The advantage of it being an app, and not a track, is that you use your ipod as an ipod at the same time, so you can listen to your own music, and the app just tells you when you need to change between walking & running, and occasionally interrupts to let you know how far you've got.
But it looks as though for the nano you have to have it as a podcast, rather than an app.
If you google ipod nano c25k there seem to be several available.
Sorry I can't help more!