Exercise is medicine. I believe that a large amount of disease (mental and physical) and other ailments would decrease in our country if more movement and less sitting were going on. The impact of exercise on depression has been examined by many researchers and results look pretty promising.
Exercise appears to improve depression more quickly than medication alone and decrease relapses. Some researchers even believe that medication can be decreased or discharged from some patients' medical plan. There is very strong evidence that exercise helps to reduce anxiety levels.
Aside from the reduction in depression and anxiety, exercise is known to evoke a positive mood and improve confidence and self-esteem.
Going out for a walk once a week will most likely not make a huge difference in the factors listed above. While some walking is better than nothing, best results occur when people exercise regularly (at least five times each week) and when they stick to their exercise regimen. Also, vigorous exercise proves to be more effective than moderate exercise.
Another interesting note is that people tend to benefit from group exercise. Not to say that exercising alone won't be beneficial. However, the comaraderie that occurs while breaking a sweat with other people in aerobics classes, cycling, or a group of runners may help to give your mood a little extra boost.
So for anyone battling the stresses of life, struggling to beat depression, fighting off anxiety, or just wanting to be a little happier, get out there and start breaking a sweat!