Don’t bother with this achievement until you’ve finished all the rally/season achievements. I got over 60km just by making my way through 3 seasons.
The only snowy stages are in the Sweden and Monte Carlo rallies, and only Sweden has 100% snowy stages. Of those stages, I recommend Karlstad (the SSS) for the purposes of this achievement. It’s 3.80km long that I can finish in less than 1min 45sec and I am by no means spectacular at this game.
The other 100% snow stages have too many sharp corners and that’ll drag your average speed down quite a bit, so I recommend Karlstad over them.