How fast should I run?

I'm a bit perplexed. I've been doing my running workouts on a regular basis. Yesterday was Week 3 Workout 3. Normally during my intervals I tend to run quickly and sort of putter out close to the end. Well yesterday I decided to change it up a little bit. Instead of starting out quickly I slowed my pace down slightly and was able to maintain that pace throughout the workout. In fact, I was feeling pretty good after my workout. So good that I ran an extra half mile just for the hell of it. So I'm not sure what the best approach is, I didn't push as hard as I normally do, but on the otherhand I felt pretty good and was able to run for a much longer time.

As far as my physical status, I'm happy to announce that I am currently pain and injury free. Let's hope it stays that way!!!