Personally I retried the first course over and over and I simply AFK'd/Idled while holding forward the left stick. You will run into trees etc. but the distance counts. You will have to do this multiple times (players have reported 7 course runs) but it's low effort
Make SURE you are not on snapshot mode during stage select from the menu as this distance does NOT count.
The Xbox tracker does track progress.
Alternatively, you can ignore this achievement as you will get it while unlocking the other Slalom achievements below