Am 60 Yrw. Have Started Walking This Week. Today I

Am 60 yrw. Have started walking this week. Today I walked two miles on pavement, one mile on High School track. I walked one lap and then climbed stairs on bleachers. Up about 30 steps/down same steps 4 times , walked another lap and then stairs. I did this 4 times. Now I find that I am having problems on the outside of my knees. Left knee is twice as painful as right knee. Hurts to walk, but is quite painful as I climb up and down the stairs at home. What could this be and how do I recover from it?