Distance freestylers use a hip-driven stroke, arms gliding long in front and legs acting like an engine in the rear. You can swim far like that. But a shoulder-driven stroke is better suited in the 50, the shoulders driving down and the legs almost rising up behind you. I still use my legs for propulsion but additionally employ them as a leveraging tool to rotate my body. Instead of just trying to move the water as fast as I can, I try to anchor it with my leg to slip around and over it. That way, I don't need to generate and expend as much power to get into my catch.
The center for all of my strength is an X axis that crisscrosses my core, from opposite shoulders to opposite hips. A line of tension runs through me from my fingertip to my opposite toe. The hardest part in training is to maintain the flexibility and strength through that X axis, through the core from the shoulder to the opposite hip. If I don't have that deep interconnection and unity, gears start flying and my swim breaks down. In sprinting, the entirety of the body needs to be solid and connected, from fingertip to toe. It's almost like reverting to the state before you l earn how to swim, when you're tense in the water.Bonus links: five books for the swim-obsessed; two of my favorite books about swimming.