Stretching should be done after your walk. Stretching cold muscles can make them snap (think silly putty) and cause injury.
Do some basic up and down squats to warm up the knees. When squatting, keep your back straight and head up, looking straight ahead. Squat down as far as comfortable concentrate on both the up and down moves to work your muscles.
Step right foot out to the side and squat, then step back into position. Repeat other side.
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