One of the toughest songs on the playlist is "Beautiful People" from Burlesque. The first time I heard the song I knew I had to use it for a class. There are four sprints during the song. The first three songs are separated by only 15 seconds rest AND each sprint gets longer than the previous. It's at a great point in the playlist that everyone is well warmed up, but still has some good muscles left. Add it to your playlist. If you're running a 5K soon, put it on two minutes short of when you expect to end, it'll push you to a PR.