Thousands of people around the region took a fitness-inducing midday stroll Wednesday to mark National Walk @ Lunch Day. Half of them walked laps around Rittenhouse Square. Will it make much of a difference? Not if the walkers are looking to lose weight fast. But there is a bigger point, said Gary Foster of Temple's Center for Obesity Research and Education. "Walking is a fantastic thing to do, because it improves your health. And it is the single best predictor of who keeps weight off and who doesn't."