For anyone who has done a long distance endurance race (10 km swim, a 70.3 Ironman or any similar long-distance endurance competition), you may have suffered what is popularly known in the Americas as the "bonk" (apparently, in England, "bonk" is something entirely different and may lend itself to embarrassing moments). It is when you completely crash, energetically speaking. It can happen any time your body is reaching or has hit an energy deficit.
This can be avoided pretty much in one way: eating and hydrating. And I'm not talking about eating just any old thing and whenever.
Immediately after a long competition, you should load up on all the carbs you lost along the way. It is of the utmost importance that you load up on all the carbs you can find. Grab those Gatorade bottles and have at least two.
Eat pasta, jelly sandwiches, baked potatoes, cereal bars or juice.
All this should be done within the first two hours immediately after finishing.