At the outset, this seems a no-brainer: it's a continuous run, done fast. The question becomes: how far and how fast (okay, 2 questions)? Here's where another rule of thumb comes into play. For most runners training for most distances, running one fifth of all miles fast is about right; some do a little more, some a little less, but it's a good starting point.
So, from my plan to run a 5:18 mile, I have about 35 miles/week training and a fifth of that is a convenient 7 miles run hard each week. The previous post detailed a workout that built to 3.5 miles of repeated 200 meters, or half the total, leaving a fast continuous run also of 3.5 miles. Because the interval run gets longer as one improves, say from 16 repetitions (2 miles) to 28 reps (3.5 miles), the FCR gets shorter, going from 5 to 3.5 miles. As the FCR gets shorter, however, it gets faster and in the example I'm using, more closely approximates racing conditions.
How do you determine the pace of this run, especially given that it gets shorter and faster? Here's where I have to give my definition of some terms: a tempo run is half of a race or more, a threshold run is a third of a race and a moderate fast run is a fifth of a race. If you run at marathon pace, you could do 3 miles easily, but at 5 miles (1/5th of a marathon) you will regard it as a marginally tough run. At 9 miles (1/3 marathon), a typical threshold run for marathoners from almost any school of thought, it's become a hard run. If you continue to 13 miles (1/2 mar.), a tempo run, you're going to need to recover from it as if it were a race.
The answer, then, is to treat a FCR as the first one-fifth to one-third of a race (preferably the latter). There is one caveat: it should not dip below 15 minutes, for it then becomes a different type of run, focusing on a different aspect of training. If my early FCR is 5 miles, I try to do it at roughly 25K pace, but will accept doing it at marathon pace. For me, training ti run 5:18, that'd be 5 miles at 7-7:20/mile. At the end of this phase, when running 3.5 miles, I'd be running at 10 mile to 30K pace (6:45-7:00/mile).
I should point out that for some runners "anaerobic threshold pace" is a real, measurable quantity. It isn't for others, including myself, for whom it is a continuously shifting range of paces. The nebulous quality of this term is demonstrated from the introduction of things like "ventilatory threshold" and the practice of many coaches, such as Brad Hudson, to use several different "threshold" paces. It's still a useful idea, but needs to be defined by each person who uses it and I choose a definition that's simple and practical and still manages to mesh with others' usages fairly well.
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