What Causes Numbness In Ankle Along With Stiffness In Calves?

I am a 60 year old male who has been over weight most of my adult life (5 10 226lbs). I have started an exercise program that consists of mostly walking on a tread mill in the early morning for 45 minutes. I have lost about 30 lbs (down from 256) over the last 6-7 months. Recently I have been experiencing a mild numbness in my left ankle/foot. My normal walking pace is 3.5 to 5 miles per hour during my morning workout. There is no pain involved and the numbness does not impair my ability to continue my walking routine. I sit at a desk all day for work (try to get on the tread mill twice during work for 10 minutes at 3 miles per hour in the morning and 10 minutes at 3 miles per hour in the afternoon. Other than stiff calf s there are no other symtioms.