Add (in category) post navigation to the bottom of single posts and throw some Genesis styles around it.

post_nav.php

<? //remove this line

// Add Prev/Next post navigation within a single category
add_action( 'genesis_after_entry', 'cd_prev_next_posts_nav' );
function cd_prev_next_posts_nav() {

	//bail if we're not on a single post
	if ( ! is_single() ) {
		return;
	}

	echo '<div class="archive-pagination">';

	// build links to prev/next post in same category
	$prev_link = previous_post_link( '<div class="pagination-previous alignleft">%link</div>', '&laquo; Previous Post', TRUE );
	$next_link = next_post_link( '<div class="pagination-next alignright">%link</div>', 'Next Post &raquo;', TRUE );

	echo '</div>';
}