It looks like this section is in Brienne's point of view, which means the narrator cannot tell us for sure what Longbough would think of the things she could tell him; Brienne does not know for sure what he would think, and we are in her head at the moment, so we don't know for sure either. But Brienne SUSPECTS that Longbough would not want to know all that she could tell, so the narrator says that she could tell him more "than he might have cared to know." If she knew for sure what he would think, then the narrator would say that she could tell him more "than he would have cared to know," or perhaps "than he would care to know."

Brienne probably had gruesome stories to tell from her own experience, of how dangerous the roads are. The kind of stories nobody likes to hear told to them. Longbough is obviously being chivalrous, and doesn't understand that Brienne knows quite well how dangerous the roads can be.