I am one of the developers of Track Runner, the only 5 star-rated Windows Phone running app. We have just come out with a big update that brings adaptive interval training and a pacer to the app along with in-depth post training analysis.
The adaptive interval trainings get harder or easier depending on your performance and progress.This way the training stays demanding. You can also make your own custom interval training which is very simple to set up. During the training the app gives you voice intstructions while monitoring your progress so you kow when to speed up or slow down to stay on target.
Similarly, the Pacer lets you set a distance and time for example 5k in 25 minutes and then coaches you through that run so you run at a constant speed and reach your goal. You can also enable negative splits to improve your race times and train energy conservation.
After the training the diary will give you details about your pacing attempt or an interval by interval breakdown of your workout in case of interval training.
If you have a Windows Phone or know of someone who does we hope you give it a try. The app is completely free of charge (no in-app purchases or annoying banners) and can be downloaded on www.trackrunnerapp.com.