A New Orleans run defense that has struggled most of the year faces a huge challenge against a physical San Francisco running game in Week 12. Two old rivals square off when the 4-7 New Orleans Saints travel to meet the 6-4 San Francisco 49ers on Sunday. The Saints are coming off a 27-20 home win over the Rams that snapped a two-game losing streak and need a win to stay alive in the NFC South. San Francisco has won three straight and is tied for the NFC West lead. New Orleans ranks 12th in total defense, allowing an average…