A picture is worth a thousand words...
Even if you're just riding for transportation, you are getting a workout. My commute to and from work gets my blood pumping for at least 60 minutes/5 days a week. And when you go out on weekends, you burn off a little of your evening's indulgences on your way back home.
Biking gives you some basic fitness that you won't get by driving a car or taking the bus. Just remember, you won't have calves like this by sitting on your ass.
However far you want to push it is purely up to you.