This question has been definitively answered in high-level triathlon forums with guys who are hella fast on bike/run AND swim and train seriously on all 3.

I'm in triathlon mode now, and I was a solid runner before cycling (ran near a 7min/mile for a marathon), and am probably equally fast on the bike as the run now (but I'm a somewhat slower runner now that I x-train.)

I can say absolutely that all the crap about x-training to get better on the bike is crap. To get better at the bike, you gotta bike. Same with the run. There is a small x-over with run to bike, but much less so from bike to run, and once you're fast on either, it's close to no overlap.

I can't stand hearing about all the plyometrics, weightlifting, running, or elliptical x-training to make you a FASTER cyclist. It's all a bunch of BS. YOu gotta train hard in the speciifc sport to get good at it.

Weightlifting in particular does nothing for almost all cycling events save short track sprints, but it keeps coming up over and over again with folks swearing it's making them better. These same folks are never racers, and never fast on the bike.