The problem here is multi-faceted:
1. Are a runner's reported mileage his/her actual mileage? Why give away all of your cards? Only that individual runner knows how much they are currently running.
2. You bring into a race ALL of the training you have EVER done not just the training you've done in the previous 1 month or 6 months. So, averages can be misleading. Just because runner A makes a breakthrough while running 50 miles a week doesn't mean the 100 miles a week they ran 3 months ago or 6 months ago or 1-2 years ago doesn't have an effect.
3. As a runner gets older they will need to reduce time on their feet. The catch here is that the large majority of runners will peak between say 25-30 years old. Right about the time father time decides to throw wrenches in the system after training at a competitive level since age 14 or earlier for some. This reduction in time on their feet ties into point #2.
4. Many world class distance runners over the ages have been thoroughbreds not draft horses. Meaning, many runners will have periods of quality training both in volume and intensity between periods of forced layoff due to sickness and/or injury CAUSED by this quality training (Ritz and Goucher come to mind). Very few world class runners will be able to train at a high level year after year for 10-15 years without getting hurt and/or sick. Interesting how many a world record or Olympic medal in the past has come after a runner was forced to take some down time (Joannie in '84 sick I believe and Clayton during the 80s would train, train, train get hurt, take some forced down time then set a record).
"US team on the track without <50 sec 400m speed."
And I argue there are far more distance runners with <50 400m speed than those with the ENDURANCE to sustain 60s speed. How many runners can run <60 for 400m? How many <2:00 for 800m? How many <3:00 for 1200m? How many <4:00 for the mile? Endurance is in part built through miles, but more importantly the ability to handle more training stress is first built through simple time on your feet.
You'll be hard pressed to find American or World Record Holders or Olympic medalists in anything 800m and up who have never spent some time over 100mpw. In the cases you do find the majority will be in the 800m-1500m range and even those will be more 800m guys than 1500m guys.
Alan