For many of my runs I listen to a awesome mix made specifically for runners called PodRunner.
What I really love is that the DJ (DJ Steve Boyett) does a mix that is usually about 40-60 minutes long and he keeps the mix at a steady beat. The length is good since my runs average 30-90 minutes.
Now what about the beat? Why is that important? Well, research (originally by the famed running coach Jack Daniels, yes love the name) shows that the optimal foot strikes per minute for a runner is 180-182 for the average runner. The great thing with PodRunner mixes is that each mix tells you the BPM (Beats Per Minute) so you can choose a mix that is suited to your running style and/or the intensity of your training session.
As for using BPM that are lower than 180-182, you may want to read up on the research by Jack Daniels in his book "Daniels Running Formulas" since he indicates that this number should stay the same no matter the intensity of the training the session. The length of stride being the indicator for how hard the training session should be.