In Reply To:
There was talk and some actual experimentation, that the Kenyans would take the world of Nordic skiing by storm a few years ago. Made sense: Put men with the greatest aerobic motors known to man in the sport witrh the most aerobic demands and you will get success. Right? Wrong! For two years two very top Kenyan runners toiled away under excellent tutalage in Finland and they were still finishing embarrasingly far behind the fields in World Cup races.
There was a great scene at the last Olympcs that the great Bjorn Dahlie skiied in welcoming a Kenyan xc skier across the line a long time AFTER Dahlie had warmed down and recieved his gold medal!
The problem has been traced to the fact that aerobic sport endurance is sport-specific. For example, it would be a mistake to think that Steve Larsen ran sub-3 hours at LP in 2001 because he had a big "engine" from cycling. He ran sub-3 there because he's a helluva good runner. He was running in his youth before taking up cycling.
Years and years training for one aerobic sport doesn't give you much of a head start when switching sports. Those who adapt quickly to multisport simply have an aptitude for the three sports.
That said, I'd bet big money that a sub-2:20 marathoner could -- with the right coaching -- develop into a helluva duathlete. From what I read of the Kenyan "experiment" in triathlon, those guys weren't doing truly dedicated efforts on the bike. It takes at least a year of 200-300 miles a week to get "real" bike legs. Not that I've ever done it; that's why I'm MOP on the bike.
I hold the belief that front-of-pack biking ability is out there to be had by nearly any of us if we put in enough time. Running and swimming are entirely different matters, and require a good bit of "gift" to get to the front.
A long-time riding buddy of mine (who was always just a little bit slower than me) "decided" two winters ago that he was going to go 40k in less than an hour (causing me no end of chuckling). His PR at that time was 2:11. He simply amped up the mileage that winter, riding 8-10 hours a week. Nothing really hard-core, just lots and lots of riding. He rode 59:20 that spring in a TT. He has now dropped back down to his old pace -- a little slower than me.