This is long and tiring(mainly cause of so many loading screens), but you don't have to race. If you've got a friend, just get to the race start(let the lights go out), make sure you're in first place, then have friend retire, then you can retire. Repeat this 35 times or enough times to get the achievement, and that's it(it's a little delayed, usually popped during loading to the next race).
Tip: Fill up all 20 spots with the same track in the track selection menu, so that it loads faster. You can also get the win 10 Online Races achievement when working on this.
I used this method with 2 computers/friend's account, and it worked. So hopefully this helps anyone else wanting to get this.