Every month we remind you of the reasons you should exercise regularly. Need two more? Your breasts. New research shows us that exercising often and at a higher intensity (RPE 6 or greater*) has a protective effect against breast cancer-findings that dovetail with the American Cancer Society's latest recommendation of 45 to 60 minutes of physical activity five days a week. To make it easy for you, we asked Tom Holland, a trainer and an exercise physiologist in Darien, Connecticut, to design a routine using intervals to keep the intensity high but also still manageable (so you don't keel over). "You'll work really hard for a short period of time then recover at a more moderate level," he says. "Cancer prevention aside, this type of training burns tons of calories, improves your endurance, and- because you keep switching up the intensity-stops you from getting bored."