1. Firstly, you don’t need to check with is_single() in single.php. WordPress will automatically recognize it for single post.
2. Secondly, breakdown your template with in parts. Such as break your main loop code in content-single.php and call it in single.php with get_template_part('content', 'single').

So you single.php template will look like below-

<?php while ( have_posts() ) : the_post(); ?>
    <?php get_template_part('content', 'single-heading');?>
<?php endwhile; ?>
<?php //sidebar widget goes here  ?>
<?php while ( have_posts() ) : the_post(); ?>
    <?php get_template_part('content', 'single');?>
<?php endwhile; ?>

And create two files content-single.php and content-single-heading.php in the directory where single.php is. Then put below code in content-single.php–

<div class="post-content">
   <!--Post content and footer goes here-->
</div>

And this piece of code in content-single-heading.php–

<div class="post-heading-info">
   <!--Post heading info like title, category goes here -->
</div>

That will make your theme code more maintainable. And for further information read WordPress codex and analyze WordPress default themes.