Check out the app Run Kaizen/Kaizen (I do their blogs). It's better than other apps for really aiming to beat PBs as it simplifies your training load, while calculating everything around your target time. It gives you a very accurate (more than other apps) race prediction. I know this sounds a bit salesy—I can't stop! My writing is normally better than this I hope (I am so tired).
I am not trying to actually sell it to you, but I use it and it's useful. There's a free trial. I'm a believer in it as much as anything. It's not my main job, but a side thing I do because I like the app. Working with Kaizen really got me into running, and I echo the statements above about it being so good for mental health. I have half an eye on the Madrid marathon in April.
The app is still young, but it's the brainchild of an uber keen sports scientist. The next update has some important new features. In a couple of updates I think it's going to be quite special.