Walking is the key to good cardio treadmill workouts. For the first four weeks, you should try for twenty minutes of walking at a pace of about 4-8 mph, depending on your comfort level. Adding ten minutes for the stretching, warm-up and cool-down, you’ll spend a half-hour each time. For the next five weeks of treadmill exercise, add ten minutes to your walking time. Do this until you are spending a total of sixty minutes on the treadmill at a time. To keep it interesting, you can change the incline slightly but adjust your speed accordingly to stay in that target heart rate range.