A gist for showing how WordPress actions and filters can be used.

race-1.php

<?php
// The starting line
start();
 
// 26 mile markers
for ( $mile = 1; $mile <= 26; $mile++ ) {
    
  if ( $mile == 13 ) // Halfway mark
	halfway_mark();
    
  // Mile marker for $mile
  place_mile_marker( $mile );
}
 
// The finish line
finish();

race-2.php

<?php
// The starting line
do_action( 'start-line' );

// 26 mile markers
for ( $mile = 1; $mile <= 26; $mile++ ) {

  if ( $mile == 13 ) // Halfway mark
    do_action( 'halfway-mark' );

  // Mile marker for $mile
  do_action( 'mile-marker', $mile );
}

// The finish line
do_action( 'finish-line' );

race-3.php

<?php
add_action( 'start-line', 'start' );
function start() {
  // Output the starting line
}

add_action( 'halfway-mark', 'halfway_mark' );
function halfway_mark() {
  // Output the halfway marker
}

add_action( 'mile-marker', 'place_mile_marker' );
function place_mile_marker( $current_mile ) {
  // Output the mile marker at $current_mile
}

add_action( 'finish-line', 'finish' );
function finish() {
  // Output the starting line
}

race-4.php

<?php
add_action( 'halfway-mark', 'give_water', 10 );
function give_water() {
  // Drink your water FIRST!
  the_hydration_station( 'water' );
}

add_action( 'halfway-mark', 'give_gatorade', 11 );
function give_gatorade() {
  // Electrolytes! It's what plants crave!
  the_hydration_station( 'gatorade' );
}

race-5.php

<?php
function the_hydration_station( $type  ) {
  $healthy_liquids = array( 'water', 'gatorade' );
    
// If $type is healthy...
  if ( in_array( $type, $healthy_liquids ) )
    echo 'Here runner! Drink some ' . $type . '!';
  else
    echo "Don't drink that! You need water!";
}

race-6.php

<?php
function the_hydration_station( $type = 'water' ) {
  $types = array( 'water', 'gatorade' );
  // Allow filtering...
  $healthy_liquids = apply_filters( 'healthy-liquids', $types );
    
  // If $type is healthy...
  if ( in_array( $type, $healthy_liquids ) )
    echo 'Here runner! Drink some ' . $type . '!';
  else
    echo "Don't drink that! You need water!";
}

race-7.php

<?php
add_filter( 'healthy-liquids', 'add_healthy_options' );
function add_healthy_options( $types ) {
  $types[] = 'brawndo'; // It's also got what plant's crave!
  $types[] = 'vitamin water';
  return $types;
}