Its not only the muscle groups that are different for running; the stress you put on various ligaments and tendons is quite different. If you are starting to run after a long period of only cycling you may need a very slow and gradual buildup. Your cardio finess may even work against you since you have to run very gently, very little, for a few weeks to build up the conditioning. It can be very frustrating if your heart and lungs are ready for a five-miler but your feet are not.