I'll play doctor here. Yes running is a great compliment to cycling. Why you might ask..well i will tell you. Running is what you call a "load bearing" exercise. It is important (especially if you are young) to put stress (mild) on your bones for them to remain strong. When you run you actually "break" down you bones and with time, rest, nutrition they are built back up stronger. Cyclists who bike tons all the time have been shown to have low bone densitys. Not to mention running is a great endurance exercise. Running can also be used to lose some pounds in the offseason.