NOTE: Conversion of previous post from km to miles.
If you want to beat your PB, you must run LONG and FAST. Running fast intervals and slow long run is not enough. Running always fast long run and never fast intervals is not enough. Training is a combination of different speeds, and, more slow is the speed, longer is the duration. If we want to see what really happens in our body, we can see that, for very little difference of speed (for example, from 4:48 per mile to 5:05 per mile) the level of lactate is very different. The type of work has different targets, the time that you use for building the same enzymatic situation is different, and the quantity of fibers interested in our run is different. Running at 4:48 or at 5:20 or at 6:00 are different type of training. So, we must put, in our training, ALL these speeds. I give you an example, for an athlete having a PB of 15:00 in 5000m :
100% of speed = 4:48 per mile (this speed is good for some interval till 2000m, using a general volume of 5-6 mile like 1000 / 2000 / 1000 / 2000 / 1000 / 2000 with 3:00 recovery, in 3:00 / 6:00, for example). The goal is to increase the ability in removing lactate from muscular fibers. This training has a direct influence in raising the Anaerobic Threshold.
105% of speed (4:48 less 14.0 = 4:34 per mile) is SPECIFIC SPEED ENDURANCE. The goal is to increase the ability in accumulating lactate. We can develop a global volume of 3 miles, using intervals between 500m and 1km (2 x 1000 in 2:50, rec. 3:00, plus 6 x 500 in 1:25 rec. 1:00, for example)
110% of speed (4:48 less 28.0 = 4:20 per mile) is HIGH SPEED ENDURANCE. The goal is to increase the ability in producing lactate. We can use this speed for a global volume of 2 miles, using intervals from 400m to 600m, example 400 / 600 / 400 / 600 / 400 / 600 / 400 in 64.0 / 1:36 with 1:30 of recovery.
Speed faster than 110 % (for example, 200m in 27.0, 400m under 60.0, or 1000 m under 2:40): have a MECHANICAL goal, and/or can work for increasing the LACTIC POWER. We can use only few repetitions with very long recovery.
When we go slower than the pace of the race, we can have the following situations:
95% (5:02 per mile): the goal is to increase AEROBIC POWER. We can use long intervals (for ex, 3 x 3200 in 10:04 + 1000m fast at the end, with 3:00 recovery) for a global volume of 6-9 mile, but also LONG CONTINUE RUN for 3-4 miles in 15:05-20:10. The ability in EXTENDING this speed can give a better base for the workouts FASTER than the 100% speed, helping in raising your Threshold.
90% (5:18 per mile): the goal is to increase the support for the AEROBIC POWER. You use this ONLY with continue run, starting from 6 miles for looking for extending your endurance till 12 miles. Running 10k in training in 33:00 for an athlete of 15:00 in 5km is very easy, but he cannot run in short time in 31:00 if doesn't become able to extend his long run till 20k in 66:00.
85% (5:30 per mile) : the goal is to increase AEROBIC ENDURANCE, that is connected, more or less, with the AEROBIC THRESHOLD. You can start with 10 miles, for going till 20 miles. THIS HAS NOTHING TO DO WITH MARATHON.