I recommend waiting until Byron Bay is upgraded to level 5, so that you can run this championship from the Goliath Circuit race. This race alone circles the entire map, is fun and relaxing to drive, and you can set the number of laps. With the addition of nine other events, you will easily meet the criteria of 100 or more miles.
Set up a championship blueprint from Goliath Circuit, located on the east side, right near the Horizon Festival Hub. Two or three laps should be sufficient. Add nine other events you'd like to race. Tweak the laps of your circuit races until you're at or above 100 miles.
10 events. 100 miles.