My coach for high school cross country and track loved to have us do a workout we called SIs (slow intervals). The idea of it is from the PAAVO training program but I think he made his own significant variations to it.
We would run 8-12 400s a little faster than 5k pace (my xc 5k was 5:10-5:15pace, so I would run 76-77s). The recovery between the 400s was 600m run at or a little slower than our pace for an easy distance run (I would run the recoveries in about 2:45 ie 7:20pace). After every 4th 400 completed we would run 1000m (which I would do in ~3:35) before going into the next set 400s.
Simply put:
-4x400m @ a little faster than 5k pace
-600m running recovery @ easy running pace between 400s
-1000m running recovery @ easy running pace after every 4th 400
-repeat for 2-3 sets, 8-12 400s total
We would usually run this workout on a Thursday with a race on Saturday after running a 3-4mile threshold workout on Tuesday. This was pretty much the fastest running that we did other than some strides.
How does a workout like this benefit you as a runner and what are its limitations? I always thought it was kind of like an interval workout except the recoveries are really long (more than 1:2 work:recovery) but that fact that the recovery is running and not standing or jogging might make up for that a bit as your are keeping your HR higher. We never wore HR monitors or anything so I'm not really sure how much my HR would come down on the recoveries.
My coach liked that these workouts ended with more mileage compared to a more traditional interval workout. Running just 8 400s this way, with a mile warmup and a mile cool down was 7.5 for the day.
Thanks,
Joe