To make physical improvements, you need to work your body harder than usual. This is referred to as the overload principle. As your body becomes more conditioned, you need to increase the frequency, intensity, or time of your workouts in order to continue improving your fitness level.
Frequency: How often you exercise. For beginners, consider starting with 2-3 sessions per week.
Intensity: How hard you exercise. For example, the pace you walk or run, the amount of weight you lift, or your heart rate count.
Time: How long you perform an activity. "Time" can also refer to the number of sets or repetitions you perform in weight training.
Exercise Component 1: Aerobic Exercise
Aerobic exercise increases the health and function of your heart, lungs, and circulatory system. For maximum effectiveness, aerobic exercise needs to be rhythmic, continuous and involve the large muscle groups (primarily located in the lower part of your body.) Walking, jogging, cycling, aerobic dance, and stair climbing are examples of activities that use large muscle groups. Activities combining upper and lower body movements such as cross-country skiing, rowing, and swimming can lead to even higher levels of aerobic capacity.
Exercise Component 2: Strength Training
Strength training is the process of exercising with progressively heavier resistance to build or retain muscle. Unless you perform regular strength exercise, you will lose up to one-half pound of muscle every year of life after age 25. Muscle is a very active tissue with high energy requirements, even when you are asleep, your muscles are responsible for over 25% of your calorie use. An increase in muscle tissue causes a corresponding increase in the number of calories your body will burn, even at rest.
For more information: Strength Training Basics
Exercise Component 3: Flexibility
Flexibility is a critical element of an exercise program but it is often overlooked. Stretching is important for a number of reasons; increases physical performance, decreases risk of injury, increases blood supply and nutrients to the joints, increases neuromuscular coordination, reduces soreness, improves balance, decreases risk of low back pain, and reduces stress in muscles.