I used to push like a motherfucker and always try to trim my times, but new information from guys like Mark Sisson has sort of pushed me away from that.
Here's the basis for what he talks about: https://soundcloud.com/artofmanliness/207-primal-endurance-become-a-fat-burning-beast
So now I adhere to his formula, which is that on longer runs, your heart rate shouldn't go above your max heart rate minus your age. This, for me, is about 149 bpm, which is a comfortable enough pace that I can mostly breath through my nose the whole time. If I'm opening my mouth, I know I'm pushing too hard.
Sisson also advises doing a lot of top-end work, such as sprinting. And that's why I do the hill sprints, which I must do more of. But I've been working with this general formula since about June, and I've found that when I DO push myself hard now, my times are about the same or a little better, but I'm not nearly as blown out afterwards. I'm trying to combined that with a newly cleaned-up diet to drop some fat and increase my endurance overall.
If I was competing, I would run more, but even my coach never particularly cared about times, even for competitive guys.