Notice that in this block of training each workout builds on the previous session. The pace is slower than the example runner's 5K pace and the volume drops in the fourth week.
Progressively increasing volume will reduce your chances of overuse injury. You should always be aware of your last training session's volume so you can wisely increase the next workout session. The pace is slower early in the season and will build up to the 5K pace as you move closer to your race.
The decrease in volume allows your body to recover and adapt to the stress and stimulus the training is putting on the body. Recovery is a key component to improving your running time.
Mid-season (8-15 weeks before A race): In this phase, your track sessions will become more intense and you'll begin to hit specific pace marks. The build up from your early season "cruise repeats" will determine specific workout pacing strategies.
The volume of these workouts slightly increases each week. You should be hitting your 5K race pace (or slightly faster) during these intervals with a 200-meter jog as your recovery interval. You're not going for an all out sprint. You want consistency. If it takes you more than a 200-meter jog to recover, you ran the interval too fast.
(Let's use the same example as above: 21-minute 5k runner—6:46 minute/mile)
- Week 1: 2 sets of 1,200 meters at 5 to 5:05 minutes; 2 sets of 800 meter at 3:20 to 3:25 minutes. Total: 4,000 meters (or 2.48 miles)
- Week 2: 3 sets of 1,200 meters at 5 to 5:05 minutes; sets of 800 meter at 3:20 to 3:25 minutes. Total: 5,200 meters (or 3.2 miles)
- Week 3: 2 sets of 1 mile at 6:36 to 6:46 minutes; 3 sets of 800 meters at 3:15 to 3:20 minutes. Total: 5,600 meters (or 3.47 miles)
- Week 4: 2 sets of 800 meters at 3:15 to 3:20 minutes; 2 sets of 400 meters at 1:38 to 1:42. Total: 2,400 meters (or 1.5 miles)
Again, notice the volume continues to increase each week and the pace is now closer to the 5K race pace mark. As fitness and speed improves, an athlete should be able to hit every interval at the desired pace.