We present TripleBeat, a mobile phone based system that assists runners in achieving predefined exercise goals via musical feedback and two persuasive techniques: a glanceable interface for increased personal awareness and a virtual competition. TripleBeat is based on a previous system named MPTrain. First, we describe TripleBeat's hardware and software, emphasizing how it differs from its predecessor MPTrain. Then, we present the results of a runner study with 10 runners. The study compared the runners efficacy and enjoyment in achieving predefined workout goals when running with MPTrain and TripleBeat. The conclusions from the study include: (1) significantly higher efficacy and enjoyment with TripleBeat, and (2) a unanimous preference for TripleBeat over MPTrain. The glanceable interface and the virtual competition are the two main reasons for the improvements in the running experience. We believe that systems like TripleBeat will play an important role in enhancing the exercise experience and in assisting users towards more active lifestyles.