I think Merritt got the right idea
“You’ve got to look for heart. The 400 isn’t for everybody. If you don’t have heart you can’t run the event. Especially an event that, it’s going to hurt. So you have to find somebody who, first, has heart. Then find somebody who has some speed, some natural speed. And what you want to do is, because the 400 is speed and speed-endurance, you have to find somebody who has speed but likes to work. And when I say likes to work – you have sprinters who can sprint, sprint, sprint all day in practice. But if you give them a 300 or a 350 they don’t want to do it. But in the 400 you’ve got to have somebody with some speed who is not going to mind, and is going to work through going out there and running a 450 and a 500 in practice. That’s what makes great 400-meter runners, the ones who put in the extra over-distance work, but also can handle that speed work.” -Lashawn Merritt
Aerobic endurance is important, but you do not need cross country to get it.