As mentioned above, it might be a good idea to have a physical that stresses the cardio side of things. Better yet, have a heart check up with a cardiologist.

There is a problem too. A stressed heart looks like a stressed heart. A physician can often mistake a heart's healthy response to intense aerobic exercise as the response of a heart to disease. So it is a good idea to go to someone who has actually seen some aerobic atheletes. Most physicians deal primarily with sick couch potatos.

You might want to vary your work outs some. Riding at such an intense level is painful. People tend to aid pain. If all of your workouts are painful, you will almost certainly give up doing them. It often helps to vary the bike workouts in duration and intensity and mix things up with some easy "joy rides".