After weeks of criticism that she had not opened up to the public or the press, Ms. Kennedy has embarked on a series of interviews. But in an extensive sit-down discussion Saturday morning with The New York Times, she still seemed less like a candidate than an idea of one: forceful but vague, largely undefined and seemingly determined to remain that way.... . She provided only the broadest of rationales for her candidacy for the Senate, saying her experience as a mother, author and school fund-raiser, her commitment to public service and her deep political connections had prepared her for...