"Every run should have a purpose" 
Running at race pace all the time will most defitnley lead to over-training and make you much more susceptible to injury.
One thing to keep in mind is that everyone is different, so there are many many different way to approach your training. Through some good research and trial and error you will find what combination of workouts work best for you.
With that said i would highly suggest that every run has some sort of faster paced work in it. May only be for 5-20% of run, but have some sort of focus. Its very very rare that any of my athletes just do long slow distance runs, because there is only so much gain from them. For example. If you are doing a long run of 10 miles, why not throw in 15s pick ups @ race pace every 15min. It will only take up about 10% of workout, breaks up monotony, but adds a different stimulus to other physiological systems (LT and Vo2 Max).
Best of luck and train smart!
Coach Jason
www.etacoach.com
