A cutdown run, also called a progression run, is a run in which you gradually or incrementally increase your pace as you go. There are infinite varieties of cutdown runs, the variables distinguishing them being the total distance of the run, whether the pace increases via a smooth acceleration or in designated increments at predetermined points, the starting pace, the finishing pace, the number of pace increases (in the case of incremental increases) and the distance or duration of running at each pace level. A cutdown run can serve any of a variety of different purposes depending on how these variables are manipulated.
Yesterday I tried a type of cutdown run that was new for me. The total distance was 24 miles. I planned to divide it into six, four-mile blocks at the following average pace levels: 7:30/mile, 7:25, 7:00, 6:45, 6:30, 6:15. The general purpose of the run was to develop specific endurance for my coming marathon: that is, the ability to sustain pace levels close to my goal race pace for nearly the full marathon distance. I was able to complete the workout more or less as planned. My actual average pace for each four-mile block was 7:18, 7:10, 6:56, 6:34, 6:27, 6:17. As you can see, I ran each of the first five segments faster than planned (the fourth segment much faster) and ran the sixth and last segment slightly slower than planned. I was really hurting in those last four miles as a result of having held too little energy in reserve over the first 20 miles.
Nevertheless, I believe the workout served its specific purpose, which was twofold. One major function of a long cutdown workout is this type is to challenge the body to run close to goal marathon pace when already severely fatigued from prolonged running. The second major function is to inure your mind to the suffering that comes with trying to sustain a fairly aggressive pace when you're already very tired. Your mind and body will be forced to face these challenges in an extreme way in the marathon itself, so it's helpful to simulate them in workouts.
Man, are these workouts hard, though! I felt like a zombie for hours after finishing my long cutdown workout yesterday. Thank heavens I only have to do one more, next week.