I did one of Skip Orem's interval workouts ("Mashup Workout 2.1") from the GetFitPod podcast. I love this guy's workouts! His voice is easy to listen to, and encouraging, and the music is fun.
The first half of the workout was challenging but very doable, and when parts of the second half were just a little too hard for me today, I was able to modify it so that I could do it and still get a great workout.
Interval training really helps with aerobic fitness & speed--I highly recommend these podcast workouts! They are great as post-C25K workouts (interspersed with "normal" runs.)