I've found that running and cycling are complimentary. Running quickly builds cardio stamina and strength in the back of the legs. Cycling builds strength in the front of the legs as well as lower back and neck stability. I've found that having strength in both the back and front of my legs makes it less likely that I'll get injured running. I used to have a lot of hamstring tightness issues, likely because my leg strength was unbalanced. Once I started mixing cycling and running, I built quad strength and I've had no issues with my legs while running. Running also helps with building ankle and foot strength and stability, making cycling efficiently a bit easier. Lastly, the back and neck stability that hours on my road bike has built makes it easier to maintain an efficient, upright running posture.