Yes, walking burns calories and you can lose weight just from walking; obviously, the faster you walk, the more you burn. A 150 lb woman burns around 55 calories per mile of slow walking and 95 calories for a mile of fast walking; a 200 lb woman burns 70 calories in a slow mile and 125 calories in a fast mile. Let's assume you weigh 175 and will walk a moderate pace; that will be about 75 calories per mile, or 375 calories for 5 miles, and you will lose a pound (3,300 calories) every 9 days, or 20 lbs over 6 months, and 40 lbs in a year.
What I would strongly suggest is that you gradually ratchet up the exercise by walking more quickly, and eventually running during stretches, or carrying light dumbbell weights, to increase the calorie burn. Also, you must be very careful not to increase your intake of calories, or else you'll just stay the same. If you put it all together, you can actually lose all 50 lbs in 6 months. You CAN do it !!!
Below are links to more info on walking for weight loss, and a guide to starting a fitness routine.