Progression runs seem to have become a fad in the running community. Or at least their popularity has suddenly greatly increased. Type "progression run" into the Google search box and you'll see what I mean. I must admit that I am more of a follower than a leader with respect to this trend. Like most runners, I've always practiced de facto progression runs, which is to say, I've always tended to run faster toward the end of my regular "base" runs and long runs. But progression runs have only had a formal place in my training since I started working with Brad Hudson on the manuscript of Run Faster from the 5K to the Marathon a couple of years ago. Thus, while there are many specific ways to practice progression runs, and I practice them the Brad Hudson way, more or less.
A progression run is a run in which the first (and usually the longer) part is completed at a steady, moderate intensity and the second (and usually the shorter) part is completed at a faster pace, usually in the range of lactate threshold pace. Hudson's rationale for progression runs is that they simply add a little bit of a challenge to workouts that would otherwise be relatively easy, and in a way that does not hamper recovery from one's most recent hard workout or sabotage performance in the next planned hard workout. Hudson likes to have his runners do them in their Sunday long runs and on Wednesdays or Thursdays. (Tuesdays and Fridays are the high-intensity days in his system). Hudson does not believe, as some coaches do, that every run should be either hard or easy. He believes that runners will absorb a higher total training load without becoming overtrained if they do one or two moderately challenging workouts per week in addition to two hard workouts (or three hard workouts if you count hard long runs) and however many easy workouts. And progresion runs represent an effective way to experience a moderate challenge.
I have my own, brain-based rationale for progression runs that does not contradict Hudson's. It goes like this: The whole reason we tend to do de facto progression runs is that our brains make us feel good and strong and eager to pick up the pace toward the end of an otherwise easy run, because speeding up will complete the task faster and because the brain's teleoanticipation mechanism is able to calculate that doing so will not be unduly stressful. By the same token, the reason we often feel sluggish from the very beginning of some runs is because the brain concludes from afferent feedback received from the body that the body is still recovering from recent hard training, so the brain makes us feel miserable to prevent us from overtaxing our bodies by running hard again before the body is ready. But with a mile or two left, our brains often lift this "artificial" limitation, knowing that a moderate acceleration to the finish will do no harm at this point. Feelings of sluggishness and peppiness during running are intelligent messages from the subconscious brain to our consciousness. It is good to heed them. Progression runs are just a way of formalizing and taking full advantage of the brain's capacity to reveal opportunities to squeeze a little extra fast running into your schedule and thereby squeeze a little more fitness out of your body.