For people still struggling to make time for exercise, a new study offers a strong incentive: You’ll save $2,500 a year. The savings, a result of reduced medical costs, don’t require much effort to accrue — just 30 minutes of walking five days a week is enough. The findings come from an analysis of 26,239 men and women, published today in the Journal of the American Heart Association. Researchers from a number of universities and hospitals around the country, including Baptis