In order to gain maximum benefits from exercise it has to be aerobic, for at least 20 minutes duration, carried out on a regular basis, 3 - 5 days a week is best. To achieve this level and regularity it is best to try to incorporate exercise into your daily life - walk to work, use the stairs rather than the lifts or escalators for example.
People often say they don't have the energy to exercise; however if you exercise on a regular basis, this actually increases your energy levels. Start slowly and gradually over weeks and months build up your exercise tolerance. Your final goal should be to aerobically walk for 20 minutes per day, 5 days of the week.
If fatigue is a problem, then start exercising in small amounts until you slowly build up exercise tolerance, this is especially true for people suffering post viral fatigue syndrome. Low impact aerobic walking built up slowly over a number of weeks isn't a problem but it would be advisable to see your doctor for a health check.
Many people think that they do enough physical exercise by gardening or walking; when asked if they exercise, many say they walk in their jobs all day and that should be enough, but even if a person is on their feet all day, an ordinary, slow walk will not supply the benefits. While these types of exercise are useful they are not aerobic. True cardiovascular exercise is performed non-stop for 20 30 minutes reaching your target heart rate range.
The Centre for Disease Control says most walkers are not walking far enough, often enough or fast enough to gain the benefits, the exercise has to be aerobic to get full health benefits.
Intensity of Exercise
There is data which shows that high intensity exercise can be stress inducing not stress reducing, so do not go above 80% of your maximum heart rate, moderate intensity exercise is best at approx 60% of maximum heart rate for your age and circumstances. Research has shown that you'll get the same beneficial results by exercising at 50 - 60% of your maximum heart rate that you would get exercising at an intensity of 80% of your maximum heart rate.
In one study researchers randomly assigned depressed participants to ride stationary exercise bicycles using 40%, 60%, and 80% of their aerobic capacity. They then randomly switched them to other conditions so all participants biked 3 times at different aerobic capacities in random order. There was a significant decline in depression under all 3 conditions. It didn't matter the order or intensity.
Duration of Exercise
Generally you should aim to aerobically exercise for approx. 20 30 minutes per exercise episode.
Frequency of Exercise
It is not enough to exercise for a couple of days a week, no matter the intensity and duration of the exercise. The benefits of aerobic exercise are short term, when you stop regular exercise you begin to lose the psychological and physiological benefits so regular exercise is a must. You should aim to exercise at least 4 5 times per week.