PHILADELPHIA – Oh, to be a bullfrog along the Schuylkill River on Monday, listening in while Gregg Popovich and Brett Brown walked and talked a few hours before Pop’s world champion Spurs – minus Tim Duncan and Tony Parker – kept Brown’s 76ers winless with a 109-103 victory. “He made me walk with him for an hour and a half today,’’ laughed an unusually amiable Popovich of his longtime assistant and good friend, who’s having a bit of a rough go