Just a note for anyone wondering how long this will take, I had completed the WRC Junior, WRC 2 and the first WRC championship (plus the Monte Carlo rally in my second WRC championship) when this popped.
If you're not concerned about completing any of the other achievements and just want to pop the mileage ones, you could run a bunch of the Spanish, Swedish, or Australian rallies that offer lots of long and fast sections. I would not recommend Monte Carlo or any of the rallies containing hairpins!