I Am 79 Years Old And Walk 2 Miles Every

I am 79 years old and walk 2 miles every day 350 days or so per year...since 1982.. I go to a gym 2 times per week and do another 2 miles on a treadmill at 4 mph with some incline. I am very active . A couple of months ago my legs began some minor discomfort and are becoming worse I think. I have continued to do what I have been doing and wonder if I am overdoing the walking bit.....18 miles per week....maybe my legs are tired and I need to reevaluate my walking habits. Otherwise, I am in good health for my age. Thank you. Jim Smith