Muscle strains??

I love to walk for exercise. However, in the past year and a half, I have started having a major problem. Every time I walk, the muscles in the front of my legs ache and burn. They get so tight that I can barely flex my foot. After I walk even a half a mile, they are so sore the next day that I can barely walk. I don’t really think it’s my shins, because it’s more on the outer side of my leg. I do have fairly flat feet. Could that have an influence? I used to walk several miles a day, and would really like to do so again. Please help!