Running has helped my biking by bringing my cardio fitness to a much higher level. I don't doubt that it's possible to work your cardiovascular system in a sustained hard fashion on the bike just like you would on a run, it just feels so much harder, and it's so much easier to slack off. Coasting - or even lightening up on the power - for a few pedal strokes just doesn't carry the guilt of taking a walk break.

Biking has helped my running by being my recovery sport. It's definitely easier for me to take an easy ride than an easy jog. I get to move my blood and flush out my muscles and enjoy the sunshine, without constantly having to nag myself to slow down as I would on a jog. That's the only way I can say that cycling helps my running.

YMMV (literally )