Not having access to or resources for such measurements, I used Daniels tables and worksheets to calculate my training paces. My estimates also agreed with Pfitzinger and Douglas suggestions for an educated guess of these paces. According to the authors, the excess energy that I wasted overstressing myself during these workouts could be better used in another workout later in the week aimed at a different biomechanical system. It sounded smart to me. With only minor differences, the authors agree on the best type of workout at each pace.
Endurance is developed by doing steady long runs.
Aerobic capacity is improved with intervals at 5K race pace sets of 600 meters to 1 mile with controlled breaks equal in time to the speed interval. Finally, leg speed requires repeats of 400 meters or less with full recoveries in between. I could go to the track for these, but more interesting options are fartlek sessions on the road or trail, repeats up a short hill or 100-meter strideouts on any straightaway. Having a good grasp of the workouts, I sat down to create a schedule leading to my goal race. Despite agreement on the physiology of the workouts and the general principle of designing phases of training, the authors were not unanimous on how and when each workout best fits into the program.