A traditional threshold run consists of a short warm-up followed by a few miles of running at "lactate threshold pace" (or the fastest pace you could sustain for one hour in race circumstances) and concluding with a short cool-down. In a threshold progression, the warm-up is greatly extended and the cool-down is removed. The purpose of these changes is to create a workout that challenges you to sustain your threshold speed when you're already tired. This makes it a great workout to use in half-marathon and marathon training. An example of a threshold progression run is five miles at a comfortable pace followed by four miles at threshold pace.
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At this point I would like to pause and answer a question that may have popped into your head when reading the preceding paragraph, if not earlier: "Isn't it bad to finish a workout without cooling down?" Actually, no. The notion that concluding workouts with a short period of low-intensity activity promotes faster recovery is mythical. Research has shown that cooling down has no effect on recovery, so it's OK to skip it in certain workouts. (Warming-up before high-intensity exercise does accelerate post-workout recovery, however.)
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In marathon-pace progression runs, the faster second segment is typically longer and slower than it is in fast-finish runs and threshold progression runs. Marathon-pace progression runs are an effective means to increase the challenge level and race-specificity of long endurance runs. Many runners make the mistake of doing all of their Saturday or Sunday long runs at a moderate pace, but once you have used these runs to develop sufficient raw endurance, they don't provide any further benefit unless you pick up the pace.
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You don't have to be training for a marathon to benefit from marathon-pace progression runs. They provide excellent aerobic support for any race distance, although you will want to use them differently depending on your specific race distance. If you're training for a 5K or 10K, marathon-pace progression runs should be emphasized relatively early in the training process and then phased out in favor of long runs that include even faster running.
More: What's the Best Pace for Long Runs?
If you're training for a half-marathon or marathon, they should be emphasized later in the training process, and they should be longer. A good peak-level marathon-pace progression run, appropriate for three to four weeks before a marathon, is two miles at a moderate pace followed by 14 miles at marathon pace.
I could write a whole separate article about how to incorporate the various progression run formats into different types of training programs. As a broad guideline, I recommend that all runners include at least one progression run per week in their training at all times. Always choose the specific format that fits best with your immediate training objectives.
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