Monster Energy Supercross - The Official Videogame 3

Select the 'Compound' from the single player section of the main menu.

It doesn't matter which vehicle class you pick, I left mine on the default and load into the compound. The map is made up of various track layouts and is quite a vast area.

I would pair this with the achievement for doing 100 scrubs as the compound seems to be the only place where that achievement unlocks. If you head behind where you spawned you'll find two sets of containers which will allow you to quickly get the scrubs done.
I then made myself a loop circuit which followed the path that the containers were located along, you can make yourself quite a nice long circuit once you've worked out your path.

This took me approx. 2 hours to get from 0 to 100 and the achievement pops in game as you clock up the 100 miles.
This can be done across multiple stints so don't feel as if you have to do this all in one go!