As runner for 35 years and still running, I began cycling more in past 6 years and at first cycling helped my running. Then as a started riding at times 100-200 miles weeks consistently my running form was a bit off and overtraining. I found you cannot do both a top level you must pick. This last August-September due to some running injuries I took off 2 months of running and cycled 1000 miles each of those months. My cycling improved much more. Now back to running only in the winter the biggest benefit for cycling is the endurance. I can easily go out and ride a 50 mile ride but to do it fast requires more specific bike work. To stay in aerobic shape running will keep you going but if you want to race and ride really fast you need to ride the bike. These days at almost 53 years old I cannot pound the pavement like I did 20 years ago but I can really ride hard on the bike no pounding. The problem with the bike is it takes more time and you can crash.

Moral is that you can go the distance on a bike if you have a aerobic strength from running but if you want to race a bike then you better practice racing on the bike. Not much different the Michael Jordan who found out rather quickly he was no baseball player based on his basketball skills.