The Peripatetic Runner                          [\ˌper-ə-pə-ˈte-tik\]: wandering from place to place
Those who have read Born to Run are unlikely to encounter much “new” information; nevertheless, the article proves to be a good read.

When I first returned to running, I found that I really only had, at most, three different paces: “very slow”, “comfortably difficult”, and “very hard”. I seldom made use of my “slow” and “hard” paces, seeing as to how the former was an uncomfortably (or, even, unnaturally) slow speed and the latter was so fast as to be unmaintainable for distances greater than 400 meters. Put them together and they are ideal for a good interval workout, but they remained entirely unhelpful for a basic training regiment. So for all practicable purposes, my training speed was primarily limited to one moderately difficult pace––a pace at which I not only ran every workout, but also, as it happens, served as my first “race pace”.
I have found this experience to be quite common among both beginner and intermediate runners. Every day one heads out the door for, say, a 4 mile run and attempts to either tie or beat one’s previous time. For beginners, this tactic often has immediate results. One notices that one’s times actually do start getting faster and after a few months, a few minutes may have even been shaved off a 5k or 10k time. But what ends up happening is that one suddenly hits a training plateau and one just can’t seem to get any faster.