This is an interesting thread but one from which it is probably impossible to draw any conclusions, because, to be almost trite, everyone is different. I remember once reading a snippet about a young man winning his first race on a butcher's bike. The young man was Raymond Poulidor, which is the salient fact. Gethin Butler is a rather special athlete. I'd hazard a guess that very few, if any TdeF cyclists could run a marathon in 2hrs 30m. That doesn't detract from their phenomenal abilities as cyclists, it just doesn't often transfer.
I was a keen runner, but, because I have one leg a lot longer than the other, I was injured more often than not and never did more than about 40 miles a week. I did a couple of marathons in my late 30s. Both were under three hours, the best a hilly (Macclesfield) 2.47. That's not a million miles away from Gethin's time but my cycling is. Thirty years later, I have done a 200 in 9 hours or so but I'm pretty sure I could never have got anywhere near a decent roadie's time, even 30 years ago. Admittedly, I just ride, I don't "train", but some people are exceptional and Gethin is one.
While I'm on, I think it was Greenbank compared an 11 hour 200 with a 6 hour marathon. Maybe if you're a much better cyclist than runner, but you are going to be much more shattered, in my opinion, after the "run" because, even walking - and you'd have to be walking to take 6 hours - you'd have been on your feet for 6 hours (less the time spent on your knees!)
I think we're trying to compare apples with pears. It's suck it and see. Almost anyone who could ride a 200 could WALK a marathon. Almost nobody could run one without a sensible build-up.
