It transfers to an extent. Lance Armstrong marathon PR is 3 hours from what I read, which is a great result for an amateur runner but totally unimpressive for a professional endurance athlete, let alone one of the best in history. I was actually a bit surprised that's the fastest Lance could do and to me it shows it doesn't cross over that well.

Not surprising at all. Vo2 in general is not great metric for elites and specificity is king. Which is why you rarely ever see elites cross train much

I wonder if running crosses better to cycling than vice versa, I think it might cross a bit better because running involves more (pretty much all) muscle groups. On the other hand there might be a lot of technique in cycling that takes a while to learn so idk.