Thanks for the link to the example, it was perfect!
Here’s what I came up with if anyone else wants to use it.
/**
* Prettify the date for populer posts
*/
function my_custom_single_popular_post( $post_html, $p, $instance ){
$category = get_the_category( $p->id );
$cat_name=$category[0]->cat_name;
$days = round((date('U') - strtotime($p->date)) / (60*60*24));
if ($days==0) {$wpp_time="Posted Today";}
elseif ($days==1) {$wpp_time="Posted Yesterday";}
else {$wpp_time="Posted ".$days." days ago";}
$output = '<li>
<div class="publish_time">'.$wpp_time.'</div>
<a href="' . get_the_permalink($p->id) . '" target="_blank" class="wpp-post-title" title="' . esc_attr($p->title) . '">
<img class="wpp-thumbnail" src="'.wp_get_attachment_url(get_post_thumbnail_id($p->id)).'" title="' . esc_attr($p->title) . '" alt="' . esc_attr($p->title) . '"></a>
<a href="' . get_home_url(). '/'.strtolower($cat_name).'/" target="_blank" class="wpp-post-category">' . $cat_name . '</a>
<a href="' . get_the_permalink($p->id) . '" target="_blank" class="wpp-post-title" title="' . esc_attr($p->title) . '">' . $p->title . '</a>
</li>';
return $output;
}
add_filter( 'wpp_post', 'my_custom_single_popular_post', 10, 3 );