I dropped my gym membership and just ride my bike too and from work. First there is the money savings angle. No gym membership, no parking, no gas, one less car cheaper auto insurance and the bike commuter benefit. (see link below).
Second there is the time savings. I used to drive to and from work as well as to and from the gym. That was mostly dead time sitting in my car. While it takes my about twice as long to ride as to drive to work, avoiding spending an hour at the gym plus 10 minutes getting to and from the gym freed up and extra 80 minutes a day.
Third I fall off the wagon less often. For me exercise it all about forming good habits. When I went to the gym generally I would go pretty regularly for about 6 to 8 weeks, then something would happen, I would fall off the wagon and I wouldn't see the inside of a gym for 2 or 3 months. But because you have to go to work linking exercise to commuting means that you will do it much more frequently. I have ridden my bike pretty consistently for the past 2 years. Getting rid of the second car, really committed me to exercising every day.
League of American Bicyclists * News