You can use this to truncate your text and then insert the link.
#====================================================================================
# truncates text based upon the number of characters and pads with a space (default)
#====================================================================================
function truncate_chars($string, $limit, $break=' ', $pad='...') {
// return with no change if string is shorter than $limit
if(strlen($string) <= $limit) return $string;
$string = substr($string, 0, $limit);
if(false !== ($breakpoint = strrpos($string, $break))) {
$string = substr($string, 0, $breakpoint);
}
return $string . $pad;
}
echo truncate_chars($string,300) . ' <a href="link_to_post/">continue</a>';