【Extend】 , 【lengthen】 , 【elongate】 ,  【prolong】 ,  【protract】  all mean to draw out or add to so as to increase in length.

Both  【extend】  and  【lengthen】  (opposed to shorten ) connote an increase of length either in space or in time, but  【extend】  is also used to connote increase in range (as of kinds, of influence, or of applicability); thus, a road may be extended or lengthened; one may  【extend】  or  【lengthen】  his stay; the power of a monarch, however, may be extended but not lengthened.

【Elongate】  (usually opposed to abbreviate ) denotes to increase in spatial length and has wider technical than general use.

【Prolong】  (opposed to cut short, arrest ) means to 【extend】 in duration beyond usual or normal limits.

【Protract】  (opposed to curtail ) adds to the denotation of  【prolong】 the connotations of indefiniteness, needlessness, or boredom.