What Could Cause Leg Pain And Heaviness That Starts After A Few Minutes Of Walking But Stops With Rest?

I am 88 years old, very active. Have always been a good walker. Belong to a gym and a few months ago just walking around the perimeter of the gym, my leg muscles started hurting and my legs felt like stumps. Once I stop walking, the pain leaves. I try walking on my street and can't walk over 4 minutes until the pain starts. I have seen my primary care doctor and he believes since my health over all is good, it is nothing to worry about. I don't want anything to be wrong but it is so unusual for me to have any kind of pain, that I can't let it go.