I maintain that running culture includes more than the marathon. Otherwise this site would be called letsmarathon.com.
Even if you want to limit the discussion to just the marathon, the US has more runners with podium finishes at World Marathon Majors in the last 12 years than Japan:
Japan, 9 (4 men + 5 women): Yuki Kawauchi, Yuta Shi*ara, Arata Fujiwara, Suguru Osako, Tomo Morimoto, Yukiko Akaba, Noriko Highchi, Yoko Shibui, and Kayoko Fukushi
USA, 12 (4 men + 8 women): Galen Rupp, Meb Keflezighi, Shadrack Biwott, Abdi Abdirahman, Shalane Flanagan, Desiree Linden, Sara Hall, Jordan Hasay, Sarah Sellers, Amy Cragg, Emma Bates, and Molly Huddle
And if you look at the last two Olympics, it’s USA 2, Japan 0.