instead of:
<?php the_content(); ?>
try and use (partly adapted from 'wp_trim_excerpt()' ):
<?php
$length = 120; //set length in words
$text = get_the_content('');
    $text = apply_filters('the_content', $text);
    $text = str_replace(']]>', ']]&gt;', $text);
$text = strip_shortcodes( $text );
$text = strip_tags($text);
$words = explode(' ', $text, $length+1 );
  if (count($words)> $length) {
    array_pop($words);
    $text = implode(' ', $words);
  } else {
    $text = implode(' ', $words);
  }
$text = apply_filter('the_content', $text);

echo $text;
echo '<a href="' . get_permalink($post->ID) . '">continue reading...</a>';
?>