As mentioned earlier, Zone mode is the fastest way (assuming you are normally into zen).
I would advise you to chose the track you score highest in (zones, not points) and just hit 'race again' when you finish until you get very bored or 100k. If you get bored, change to another good track for you and continue.
I would recomend occasionally swapping between your top 3 tracks (most likely Moa Therma White+Black will be in there)
I have: Assegai at 100k
the other non-DLCs at 50-70k
the DLCs at 6-8k (I've not had them that long, okay)
This is in just over 90 hours of play time (about 3 hours (I'd estimate) online (which doesn't affect loyalty levels unfortunately))