Results also suggest that within a given risk category, physical
fitness is associated with a greater than 50-per-cent lower risk of
mortality. In this study, physical fitness was four to five, 30-minute
segments of activity per week: equivalent to walking 130 to 138 minutes
per week.

http://debunkbigpharma.blognation.us/blog/_archives/2005/9/4/1198620.html