I recently went to see my doctor for my annual tune up. She was pleased that I exercise but felt I needed to pick up the pace. I currently walk about half a mile five to six days per week, admittedly at a slow pace. She checked my heart and said it would be safe to pick up the pace in order to tone the nervous system's affect on my heart. What did she mean?

She may be referring to the parasympathetic system, which controls many body functions, including the heart. Evidence suggests that a well "toned" parasympathetic system may prevent heart problems such as heart attacks, angina, congestive heart failure, and sudden cardiac death.

One study found that men who exercised aerobically expending less than 1,000 calories per week did not seem to have as good a variable heart rate. (This heart rate variability indicates good parasympathetic tone.) To meet that 1,000 calories, you would need to walk briskly 2.5 days per week. The hardest part is getting out there every day and doing it--the part you're succeeding with. Now you just need to gradually increase your time and pace.