TOKYO—Katie Ledecky’s teammates are already piling up Olympic medals. Now it’s her turn, and she wants some hardware of her own. Ledecky opened her Olympic program as the top qualifier in the 400-meter freestyle on Sunday night, setting up a showdown with Ariarne Titmus of Australia as the American star tries to defend her title. Ledecky advanced with a time of 4 minutes, 0.45 seconds. Titmus was third-quickest at 4:01.66. The first of their multiple showdowns in Tokyo comes in the final on Monday morning. “It’s going to be a great race with Ariarne,” Ledecky said. Ledecky will be looking to carry forward the momentum her teammates built on the first day of the meet, when the United States earned six of 12 medals Sunday. “Definitely feeling the energy from our team,” she said. “Really great session this morning, so just trying to feed off that and get my meet …