Look at it this way. You need to exercise somehow. You could drive to work and then go to a gym at some other time of day. Some rough number guesses:

30 minutes to drive to work
60 minutes to cycle to work
15 minutes to drive to gym
60 minute workout required at gym

If you cycle to work, the time cost is 30 minutes, but you get a 60 minute workout. You're saving time!

You don't have to cycle to work every day to take advantage of this. Cycling to work only on some days is fine. As it is now, I cycle to work only one day a week, and I'm hoping to increase that. In fact, tomorrow may be my second day of cycling this week.