Cycling is a very good for of cardio. For cardio training you want to pedal at a high cadence 70-90 rpm's with a lite pressure on the pedals. That will use your slow twitch muscles that use fat as a fuel and you can go for a very long time. That will tone your muscles, to make them large you want to use a higher resistance and a slower cadence. That will use your fast twitch muscles that use glycogen as a fuel and wont last as long and will need a day off to recover.
You can mix it up with interval training just like you do for running.