Dear AAGermany,
Follow the advice of the Brain Trust here on FT and those 400K AA miles will come to you much faster than you could ever dream. Such has happened to me!
When you get back to the US, study the various other ways to earn miles including Kellogg's certificates (off specific cereal boxes), FTD.com orders via AA.com, AA.IDINE.com and others.
The folks who plan most of my flights are bargain hunters and often fly me on airlines other than AA, but I am still racking up AA miles. For personal travel I lean hard toward AA.
It's a fun game to play. Plenty good benefits also!
A bit early, welcome home!