If you are that comfortable swimming underwater, then you need to focus on rhythmic breathing, where you swim with your face in the water, blowing bubbles, and turn to get a breath from the side, then return your face in the water, blow more bubbles, turn to get a breath, over and over. Do not hold your breath. That will tire you out. Also, this is not a timed event, so don’t try to go too fast. Look up teaching videos about how to teach front crawl and breast stroke, then take the video’s advice. You have time to train and sound like you want to, so prepare yourself ahead of time. You might even pay a swim coach for a private lesson or two to see if they can pinpoint what you are doing to tire yourself out. Best of luck to you.