How far do you run? How often do you run? What shoes are you using now? Are you flatfooted? Count on questions like these from the salespeople at Runner's Den. Selling you the right pair from the Den's vast selection is serious business to these clerks, most of whom are runners themselves. (One salesman has logged more than 100 marathons and teaches a community college class in marathon training.) These guys know the heartache (or leg-ache) that comes from shoe-related injuries, and don't want you to end up in, um, those shoes. They'll even make you do a test drive with an outdoor jog or a quick spin around the store. So runners, take your mark and lace your shoes.