What are some good techniques for swimming under water for long periods with no breath?

I recently posted this but I wanted to rephrase it a little. Also SUPER "CHARLIE" SCOUT if you read this please comment on this again, I liked your comment. I am trying to swim 50 meters under water with just the original breath I took to start the swim, I relax and take a deep breath and I dive deep to the bottom of the pool to expand my lungs, but I struggle to just get to 25 yards. I am training for the Navy Seals and that is one phase I have to do is swim 50 meters without any breath, I have plenty of time to train, I want to start learning now. Any techniques anybody knows for prolonging your energy and oxygen in your body to make it to farther lengths.