I’m sure that many of you who know me or read my blog will know “my” answer to this. Here are some great words of wisdom from Matt Fitzgerald as seen in his recent book, 80/20 Running: Run Stronger and Race Faster by Training Slower
It is natural for runners to want their pace to compare favorably against their normal pace in all kinds of workouts, even easy runs. This disposition seduces runners into pushing a little harder in runs where they should not push themselves than they would if they were not paying attention to the clock. A single lapse of this type does little harm, but a consistent habit of pressing the pace in designated low-intensity runs hampers progress by creating a burden of fatigue that is carried throughout the training process and lowers performance in the workouts that are intended to be more challenging. Establishing individualized target pace zones for different types of workouts, including easy runs, offers a way of avoiding this problem, but it only works if those zones are respected. As I stated earlier, it’s been my observation that runners have a harder time obeying “speed limits” in low-intensity runs than they do heart rate limits.
So in summary, adding speed and tempo runs into any training program is obvious an ideal way to improve your speed. But, running with a hard effort daily will without a doubt create irreversible fatigue that compounds as a result of lack of recovery.