I started brisk walking 5km/day for 38 days, but instead of gaining energy and stamina i m feeling more lethargic & lack of energy. don't know why?

Too much, too soon?: You may be overtraining if you are exercising every day. I would consider alternating your workouts (some biking, some walking, elliptical and/or swimming) with a max of 5 days/week. You need some recovery days. Also, look at your diet & sleep. Are you getting enough calories and nutrients. Are you trying to lose weight, and if so, have you done so? Losing more than 1 lb./week could be too much.