I believe in running two hard workouts per week "and a bit more." If one does two hard runs per week, one can expect steady gradual progress, as long as neither of the runs becomes so difficult that one cannot recover in time for the next one. It's possible to accelerate improvement by adding a moderate run each week - a pattern of: hard, easy, moderate, easy, hard, easy, easy. Unfortunately, this greatly increases the chance of getting injured. Training thus becomes a continuous balancing act, trying to do "a bit more, but not too much more." You should keep in mind this rule of only two hard runs per week when I get to the late stages of training, when it looks like I have a hard run every day; it is these ideal-week training schedules that get published for elite runners and they need to be understood in the context that some of the workouts that seem difficult are either occasional, unimportant, over-stated or simply not done.
In the phase of training where I have the EI and FCR workouts I described in the previous posts, I also include "a bit more" in the form of an explosive speed workout of steep hill sprints. Right now, several people reading this are thinking I got the idea from Brad Hudson, but he got it from Renato Canova, who got it from Brooks Johnson, who probably was influenced by Percy Cerutty, who - though he would claim he invented everything - probably was influenced by the Finnish runners of the 1930's.
The workout is a total of 30 seconds of all-out running, 3-4 repeats of 7-10 seconds. When I first do these, I do them on the steepest hill I can find; when I don't feel I'm improving, I move to a less steep hill so that I can run faster; eventually I move to flat ground (and late in this series, I'll address downhill sprints).
The first reason for doing these is as a check on the two hard runs. Because one is starting to do speedwork, sometimes temporary gains seem to be made at the cost of poor form, which will lead to injury over time. These sprints will accentuate any imbalances or inflexibility and let you know what you need to work on - for some new to these, the form problems that are minor at lower speeds can cause sudden injury when sprinting. One must approach these carefully! Paradoxically perhaps, these same sprints seem to have a protective effect against injury when done properly, as the hill decreases the forces on the legs on landing.
The method for doing these is straightforward. Building into the sprints with "a flying start," that is, not from a standing start, avoids the sudden jarring that's usually the cause of injuries when doing these. One accelerates as quickly as possible, stopping when one can no longer accelerate. This workout, though anaerobic, should never lead to breathing hard, as it uses creatine phosphate for energy and doesn't produce lactic acid. The recoveries should be complete, as long as possible, ideally spreading them evenly throughout the run (in most systems, these are done in a bunch - which is admittedly convenient - and at the end of a run). If done correctly, these are actually fun and I find the brief bursts of speed tend to carry over to the rest of the run, causing the whole to be done slightly faster.
There is no need to try to improve this workout by running more repetitions or doing longer sprints (top sprinters would go longer, but that's not my audience here). Improvement comes with increased speed; some GPS units display an instantaneous top speed, which can be useful here, if the unit works in the shadow of the hill, where they are notoriously inaccurate.