Don't disagree.
The top marathon runners are top pros as well mind. And the top guys are only really capable of running 1 or 2 a year.
I think that a marathon at all out marathon pace takes far more out of you than cycling a stage of the Tour.
As I said though, if there was an equivalent Tour type race in running though, that pace would need to come down substantially. Tour stages aren't all out efforts all the way through, albeit the attacks are probably at a greater effort than marathon pace effort.
That's why I don't really think you can equate tour stages to marathons. Apples and oranges.