Old Sub 4 makes some good points, especially about transitions. Over and over I've witnessed runners getting injured while transitioning from base-work to interval work (intensity work). That's why base work should include some easy short-rep workouts that aren't horribly glycolytic.
Here is a thought, I'm not disagree or agree just stating a view point.
The whole idea of "base-work" I think is flawed. Not only do many runners get injured but it is my experience that you lose some of the benifits from your base. It has been mentioned on here before, but I believe that there is much to be learned by distance runners in the methods of block periodization.