Why Is My Leg Painful After Walking 200 Yards?

When i walk about 200 yards i get pain in my left leg ,my doctor sent me to Hospital and i had an operation on my growing but i still get a pail when i walk about 200 yards my leg it gets very very painful and i have to stop have a rest and start again walking the pain is on the lower part of my leg i think is a muscular pain.However i am 70 years old and i never had any sickness of any kind.