The bottom line is that you do not need to exercise at high intensities to achieve significant health benefits . There is a higher incidence of injury associated with higher intensities of exercise and you may find it hard to adhere to an exercise program if you are constantly exercising at an uncomfortably high intensity. This does not mean you need to exercise for 20 minutes at a time. Studies have shown that 10 minute bouts of exercise accumulated throughout the day are equally effective in improving cardiovascular fitness as is continuous exercise for equivalent periods of time. Thus you can do 3-10 minutes bouts of exercise during one day and achieve the same benefits of performing 30 minutes of continuous exercise.
Duration is dependent on the intensity of the exercise.
Resistance training is equally as important as aerobic activity and should be an integral component of your fitness program. Resistance training is important for enhancing muscular strength, muscular endurance, maintaining fat-free mass, and for maintaining bone mineral density, to prevent the onset of osteoporosis. A minimum of 2 days per week, up to 3 or 4, with at least 1 day in between each workout. Similar to aerobic exercise, resistance training needs to start out at a low intensity when beginning the program and can gradually increase as your body adapts to the new work load. Technique is very important to prevent injuries and ensure efficient movements.