As an athlete, one of the workouts that I always seemed to struggle with was tempo runs. I am not sure what it was about tempo runs but these were workouts that I always dreaded and struggled with as an athlete. Considering that my main event was the marathon, doing tempo runs should have been one of my favorite workouts. Greatfully, I had a coach that understood my dislikes for tempo runs and worked around this by introducing cruise intervals into my training. Now this did not replace the need to do a tempo run, but it was a good alternative early in my marathon base phase.
The workout is 8x1200m at threshold pace with 1 minute recovery. This workout was primarily done based off of heart rate, and was always done in a grass park to ease some of the pounding from all of the weekly miles. If doing this workout by heart rate, I would recommend a heart rate of 60-75% of max heart rate.
The idea is to keep this workout at a threshold pace. This workout is great because the intervals are so short and the recovery is just long enough that it allows you to run at a pace that is faster than your normal threshold pace. By doing this on a grass field, it allows you to use muscles that you might not use all the time while running on the road. As mentioned above, this also allows to avoid some of the pounding that you encounter on your daily runs.
The best time to do this type of workout is during your early base phase of training. I always found this workout helpful when I was either coming back from injury or after taking some time off from training. Breaking a threshold type workout up into 8 intervals is easier than having a 6 mile tempo run staring you in the face.
In order to maintain this workout as a true threshold type run, you will want to make sure that the intervals are no longer than 5 minutes. If it will take you longer than 5 minutes to run 1200m, you might want to consider making the following changes. Either change the workout from 1200m to 800-1000m, whichever one will keep you under the 5 minute mark, or you can simply use a time based approach. I often would go off of time when I had to do this workout on the treadmill. If this is your first time doing this type of workout, I would recommend cutting the intervals down to 5-6 and work your way up to 8 intervals.