NEW YORK — Relax. That’s the key to healthy living, and this fall season designers are dressing the part, as casual weekender looks are becoming a staple in many collections. Last year, American women spent more than $26 billion on casual sportswear and a majority of them said they now buy more casual clothes than career outfits. Casual apparel has become a major component of specialty stores like Gap and Old Navy to national chains such as J.C. Penney and Kohl’s and mass giants Wal-Mart and Kmart. Here, Easy Spirit’s cotton corduroy jacket and skirt and Juicy Couture’s cotton velour sweater.