Why does brisk walking causes pain in my legs?!

Best Answer - Chosen by Voters

Your muscles are working out. No pain, no gain. =)

as you use the muscles they burn glucose and... basically,walking briskly, if you don't warm up/raise heartrate will draw more energy than you can produce with normal oxygen transfer, the start to anaerobically burn glucose instead, you get a build up of acid in the muscle which is toxic to the muslce tissue, causing the pain as a trigger to prevent you from actually harming your muscles seriously.

Assuming its the same kind of pain as you would get if you did sprints for a while

Sounds like you need more stretching; each day.
Our bodies have a covering over our muscles that holds everything together. When we stretch, we make the covering more flexible. So when we walk, less pain.

Need more info. Do you stretch before you walk? Are you geriatric?