It depends on what you're trained for, but for me personally it's more like a 10:1 ratio (in terms of distance). I don't run much-- usually one 3 mile run each week. That run at a pretty slug-like pace feels about the same as a
really hard 30 mile ride. Further, I could probably go ride 260 miles tomorrow, and while it wouldn't be easy, I'd most likely finish the ride without hurting myself. However, I highly doubt that I could go out and run a marathon tomorrow without doing some serious damage.
Incidentally, when
Lance Armstrong ran his first marathon in 2006 he said it was the "hardest physical thing" he had ever done. His time was 2:59. (no word on whether he was doping when he did it.)