Does Cycling Improve Running?

I don't have any direct evidence, but if I had to guess I doubt any of the African distance runners that dominate distance running today do very little if any cycling.
That does not mean, however, that cycling can't help your running and that you should not use it.

Former Olympian Ed Eyestone published a study in 1993 that is summarized as follows:

This study compared water running, cycling, and running for maintaining VO2max and 2-mile run performance over a 6-week training period. Thirty-two trained subjects between the ages of 18 and 26 were evaluated for maximum oxygen uptake (VO2max) and 2-mile run performance. Subjects were stratified by a 2-mile run pretest into high, medium, and low performance levels and then randomly assigned to water running, cycling, or running training. The three groups trained with similar frequency, duration, and intensity over a 6-week period. After 6 weeks of training, all of the groups made a small but statistically significant decrease in fitness (VO2max), but no change in 2-mile run time. However, there were no differences with respect to either training modality or pretraining performance level. It was concluded that over a 6-week period, runners who cannot run because of soft tissue injury can maintain VO2max and 2-mile run performance similar to running training with either cycling or water running.

This means that least on the short term, you can maintain good running fitness through cycling.

Besides the obvious (increasing quad strength, higher VO2, building aerobic enzyme activity) I've found that by focusing on pedaling technique, cycling is a good way to increase ankle flexibility and range of motion. According to Better Training for Distance Runners by David Martin and Peter Coe, "ankle flexibility seems best displayed...by African runners, particularly those who ran barefoot as children." Pedals with clips allow greater use of ankles than you get running, and it makes sense that ankle flexibility can help runners generate additional force at push-off.

Uphill high intensity intervals and easy spinning recovery rides are 2 examples of how you can use cycling to improve running. I'd plan to use cycling to replace easy run days, or as a second workout- going easy if you ran hard, or hard if you ran easy. Pretty simple.

Check out this article by Brendon Walters that cycled across the country and followed up with a 16 minute PR in a marathon on only 1 month of run training.