Take a look at this article: The Athlete's Heart. Looking at your exercise regimen, I think you are more of an athlete than you realize. Just biking the 8 miles a day has you putting on at least 40 miles per week of aerobic exercise.
The article states that with a heavy aerobic exercise regimen, the heart responds by lowering its heart rate, but increasing the volume of blood in each beat. With anaerobic exercise (weight lifting, other high intensity sports), the heart walls thicken to increase the blood pressure during exertion. These are normal affects, and the changes occur within the first 4-6 weeks of training.
Also according to the article you should consult your doctor if:
- A history of fainting for no apparent reason, especially if the fainting occurred during or immediately after exercise.
- Symptoms of an irregular heartbeat, palpitations, skipped beats or fluttering heartbeat either when resting or exercising.
- A close blood relative who died suddenly before the age of 55, or a family history of early coronary heart disease, high cholesterol, Marfan's syndrome or enlarged heart.
- A family or personal history of seizures.
- You are a male over age 40 or a female over 50, the American College of Sports Medicine recommends that you have a medical evaluation before you begin an exercise program.