Trackhead,
I don't know thru what a system of training they arrived to those results. In Kenya, the 90% of training for young runners is "natural training" with aerobic direction. So, they run long run easy, 20 min hard, some repetition not very hard (no longer than 400m) and some fartlek (normally using 1 min fast / 1 min easy). They don't seek any intensity. So, they are able running 13:20 and 3:35 only with general and basic training. THEY HAVE YET TO START THEIR REAL TRAINING.
Instead (but may be that I am wrong, I don't know), I've the sensation that US runners use a system based on intervals, anaerobic training, very intense workouts. This type of training cannot be used like normal system, because permits to reach in short time good results, but destroys your body instead to build it.
Without a big base (Lydiard was the first, and you always remember him) is not possible to improve in quality and to consolidate your level.
I want to give an example.
There is an athlete training 4 times per week, using the same schedule. When is in his top shape, his week is the following :
Mon : 30 min easy + 15 x 400 in 63.0 recovery 1 min
Tue : 40 min easy + speed (5 sets of 4 x 100m in 14)
Wed : Rest
Thu : 10 x 1000m in 3:00 rec. 2 min
Fri : 50 min progressive run
Sat : Rest
Sun : Race (3-5 km)
The total mileage is about 40 miles per week.
This athlete goes running a 10000m, and is able running in 31 min. SO, SOMEONE THINKS THAT THE RELATION BETWEEN 10x1000m AND THE RACE IS : INTERVALS AT 3:00 = RACE AT 3:06.
Next year, you mantain the same schedule, BUT EVERY DAY YOU GO FOR AN AEROBIC SESSION, so your schedule is :
Mon : a) 1 hr moderate b) Like before
Tue : a) 40 min easy b) Like before
Wed : 1 hr with last 20 min hard
Thu : The same (only one session)
Fri : a) 1 hr 10 min moderate b) Like before
Sat : 1 hr at moderate pace
Sun : Race (5-10 km, for example no more short cross, but long distance)
The total mileage is now about 85-90 miles per week.
Nothing changed about intervals, NOR THE NUMBERS OF REPETITIONS, NEITHER THE DURATION OF REST, NEITHER THE SPEED.
You go running a race of 10000m and are able running in 29:50, FASTER THAN YOUR INTERVALS OF 1000m.
So, now the relation between 10x1000 and the race is :
INTERVALS AT 3:00 = RACE AT 2:59.
What did it happen ? That YOUR SPECIFIC TRAINING OF INTERVALS NOW IS ABSORBED INSIDE THE VOLUME, and its output is better than before.
SO, THE IMPROVEMENT IS NOT IN RUNNING ALWAYS FASTER, BUT IN A CORRECT BALANCE BETWEEN RUNNING LONGER, INCREASING VOLUME, AND RUNNING EASIER AT THE SAME SPEED.
Your body and your mind are full of energy, not always exhausted. Thru too many intervals, you finish in short time your nervous fuel, and your tank can be empty for long time.
But, I repeat, can be that my impression is wrong : I don't know the reality, mine is only a supposition.