Starting to ride for fitness is a common introduction to cycling with many people riding to work and swapping their car for peddle power.

Regular cycling will not only improve your fitness, it will:
Help loose weight
Define muscle
Strengthen your heart
Strengthen your back muscles reducing back pain
Reduce knee joint pain by encouraging nourishment of the cartilage though low impact use.
Reduce high blood pressure
Battle obesity
And fight some forms of Diabetes.

Most car journeys are very short and can be completed on a bicycle, 2.5 miles can be ridden easily in 15 minutes, riding 30 minutes, 5 days a week (back and forth to work for example) can burn off around 10 KG of fat a year.
Getting out into the fresh air and being active also helps relive stress, improve your mood and self esteem especially as you are in the knowledge that by cycling and not using your car you are doing your bit to help the environment.