The biggest mistake I see with both experienced and newbie runners of all ability levels is that they run too hard on their easy days. Following a quality training day, our body needs a break from hard training for at least one, and possibly, several days, depending on how hard the quality workout was and how fast the individual runner recovers. In general, a younger runner will recover more quickly between hard sessions than a more "mature" runner. The key to improving as a distance runner is to stress your body with a quality workout, run at a comfortable level (easy or medium effort runs) until you're relatively fresh, then hit your body with another quality session. Training, in general, is really that simple. Stress + rest + stress + rest, etc., etc. = improvement.
The single greatest predictor of distance running improvement is CONSISTENCY. If we constantly thrash our bodies without sufficient recovery, we get injured or over-trained and are forced to stop training. If, however, we follow a hard training session with lighter runs until we're recovered, then continually repeat that process, we can't help but improve. Many (most) runners who join the Earth Drummers Racing Team come in with the philosophy that the harder they run on a daily basis the faster they get. Soon, however, they learn that if they follow the prescribed training plan, complete with recovery paces that they previously considered unacceptably slow, they reach levels of performance they thought were unattainable. The main reason for this is that running more comfortably between hard workouts allows them to perform better on the quality
training days. And if we can hit the goal paces on the hard days, we can do the same on race day!
So, I encourage you to give this a try. Simply try running at a pace that feels easy for a couple of days following your hard workouts and see if you don't enjoy training more and feel better in hard workouts and races. Let me know how it goes!