I am 45 and run about 45 miles per week. Recently for reasons that are irrelevant I did a lot of walking around the city (in addition to my normal running), on the order of 15-20 miles a day for four days in a row. My legs, the calves and lower quads in particular, had a lot of muscular soreness those days (normally I don't have much if any soreness). I wore various different shoes. Any explanation for that? I suppose the answer is just that walking uses different muscles than running, but I'd be curious if anyone had anything to add.