Biking is such good cross training for running that if you are running, say 6 or 7 days a week, your running performance might well improve by switching one of those days to biking; however, don't try to replace your long run. Replace one of your easy run days with your bike ride or even replace a hard day with a bike ride, but in that case the bike ride should include greater intensity.
This is probably MojoforRunning.com's favorite electronic device. I love using the metronome to work on running form by improving cadence. Remember, ideal cadence is approximately 180 food strikes per minute at any pace.