I agree with MichielR. Are you sure it is really worth it to go for FTL? With 12.000 miles so far you do not seem to fly so much. Just use the money you safe on a potential mileage run and rather spend it to upgrade to C whenever you find it relevant or get a nice offer to upgrade.
You could qualify with segments as suggested by Lack but this can be quite time-consuming and "painful".
Your options also heavily depend on where you are based. 18.000 are easier to be earned in the US market than in Europe. But then again, I would rather safe the money and spend it on the flights you have to take anyway.