I haven't done much yet with podcasts, but along those lines, I REALLY like a cycling CD I bought and loaded onto my iPod. It is the Zooming CD from Ian Jackson. I ordered the set, which also has audio for both walking and running programs. I use the Zooming workout the most (it is specifically for cycling).

http://www.breathplay.com

I also bought a Spinerval audio only workout that I put on my iPod. It is a 45 minute high cadence interval workout. I think it is good.