The Collaborative International Dictionary of English v.0.48:

Lengthen \Length"en\ (-'n), v. t. [imp. & p. p. Lengthened
   (-'nd); p. pr. & vb. n. Lengthening (-'n*[i^]ng).]
   To extent in length; to make longer in extent or duration;
   as, to lengthen a line or a road; to lengthen life; --
   sometimes followed by out.
   [1913 Webster]

         What if I please to lengthen out his date. --Dryden.
   [1913 Webster]

WordNet (r) 3.0 (2006):

lengthened
    adj 1: drawn out or made longer spatially; "Picasso's elongated
           Don Quixote"; "lengthened skirts are fashionable this
           year"; "the extended airport runways can accommodate
           larger planes"; "a prolonged black line across the page"
           [syn: elongated, extended, lengthened, prolonged]

Moby Thesaurus II by Grady Ward, 1.0:

33 Moby Thesaurus words for "lengthened":
   dragged out, drawn, drawn out, drawn-out, elongated, extended,
   interminable, languishing, lasting, lingering, long,
   long-continuing, long-drawn, long-drawn-out, long-pending,
   long-winded, marathon, overlong, prolongated, prolonged,
   protracted, pulled, spun out, spun-out, straggling, strained,
   stretched, stretched out, stretched-out, strung out, taut, tense,
   tight