assuming you don't have any fat left on your legs to lose (and to be honest if you do this you'll probably just increase definition), you aren't going to slim your legs down unless you lose the muscle you have, which probably means stopping exercise, in theory you probably wouldn't need as muscular legs to run more slowly but it will take a long time to lose the muscle you have if you're still using it a lot
I'd second other advice, muscular legs probably look much better than slightly slimmer legs with no muscle tone