Originally Posted by MichaelW

Unless you are a competative sportsperson, then any riding will give you enough excercise, that scientific fitness stuff is not critical. If you concentrate on having fun, fitness will take care of itself over the long run. Don't expect to become "cyycling fit" in a week or 2, it takes at least 3 months.
As a general rule, an aerobic fitness base is developed by riding long, slow miles, at least 40mins of steady pedalling. Sprint and climbing fitness is something you need to develope on top of an aerobic base.