Hi! A walk, anytime of the day is very good. Its best to walk on a walking track in a park or ground wearing proper walking shoes. The surface and shoes are important to prevent joint and accidental injuries.
Walk for atleast 30 minutes at a brisk non stop pace.
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In today's city living, all areas are polluted with dust and smoke. If you walk in the evening, pollution would be at peak. Instead, if you walk in the morning, you will have a serene atmosphere, clean environment and healthy air. It is more prudent to walk in the mornings, preferably in a ground. That would fresh up your lungs and keep you healthy.