This doesn't need to be done in Career mode, but can also be done in Freeplay / Custom Championship.
So just create a Custom Championship with the first car class in the list (which contains the DS 21), add enough stages for 66 km and then select the DS 21 as car and do the stages. The result doesn't matter. If you have to retire you can just start over, you won't lose the progress of the completed stages.
The achievement tracker will update with the current progress after each stage.
As the DS 21 isn't the fastest car and the Monte Carlo stages aren't the fastest ones this will probably take you around one hour to unlock.
EDIT: at least in version 1.3.0 there seems to be a bug where you can make it count the kilometers of the stage you drove twice! I did this on "Vallée descendante" and when you get to the finish then do NOT drive to the marshal but instead stop the car just after the finish line. Apparently the game counts your km for the stage once you pass the finish line and then again after some kind of timeout (the stage will still end if you stop after the finish line and do not drive to the marshal after a few seconds).
EDIT 2019-09-18: According to the comments it is unclear whether this still works in the newest version. You might have to reverse after crossing the finish line and it might not always work. Look in comments for details.