If you ever get into competitive sports, especially solo sports, you'll know how important mental endurance is. At some point, all you'll feel is random pain everywhere and the only thing carrying you along is your stubbornness to keep your pace and finish.
Depending on the sport, the kind of torture you'll experience will vary. In the case of swimming, if you're going all out for any of the longer distances, you're just slowly suffocating yourself. This is of course on top of the immense amount of pain you'll be experiencing.
The experience does however make finishing all the more satisfying, especially if you're proud of the results.