First "Tri" Swim Approaching - Thoughts

So yeah, my first "Tri" swim is approaching in a little over a week. Why is Tri in quotes? Because it is only 400 meters. Seems easy enough. And it is in a pool. That seems nice. But here's the thing. I haven't actually been able to swim 400 meters in one go yet. My arms just run out of steam.

Why do I keep seeing this image in my head when I dream about this upcoming swim?