I think Thompson's bests were around 14:10 and 29:20, and Dibaba and Defar are not that far from those times, especially in the 5000. Dibaba has quite a bit more leg speed than Thompson ever had, and that is not to detract from the accomplishments of the Luton man.
If Dibaba and Defar ran a super half marathon at their track shape best, I think they would approach 63 minutes. Going from that, they should have the capacity to run the marathon in double that time, + 8 minutes, which would give them a 2:14 time. With more specific marathon training, they might be able to go faster, and again, I am looking at 5 minutes per mile as being a possibility for those top girls.
The Japanese girls are great marathoners, but they will always be handicapped by their lack of leg speed. They do not have the power to weight dynamism that the Ethiopian girls have. Essentially, the Japanese runners are products of a tremendous training and work ethic, which focuses on time trials, repeated - until they can run prescribed pace with their eyes closed. They do more exact simulation of what they will face in races than any other nations.
I recall visiting Barry Watson, the South London school teacher, who won the British marathon trial in 1976 (ahead of Jeff Norman and Keith Angus - two northerners). He was in top shape (at around 2:15) but he could not run much faster than 29:30 for 10k track at the time, and did in fact do a time trial at Crystal Palace before the Olympic race, in which he recorded a mid 29 minutes time for 10,000 and he was all out running that. This was in conjunction with a AAA all comers meeting.
Women, compared with men, run better over longer distances relative to their native track speed. This may have a physiological reason, as women tend to have a better way of utilizing diminishing reserves over a long distance.
That is why the record for super distance Channel swimming is often little different between the top women and men.
Recently in the French 100km distance race in Millau, a woman finished 3rd overall, and went through the marathon distance not much slower than her best time....and kept going!
The point is, one should not just look at track speed when discussing predictions for the marathon. There are other variables which should be taken into account.
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