Easier than it sounds. Go into 'Custom Championships', set up a single stage championship selecting one of the longest available (~6.6 km). Set difficulty to the lowest setting (0 = very easy), and select whatever car class you prefer.
Using the R5 class, I used one of my vehicles that had the highest engine performance modes unlocked. With each waypoint, I was able to pick up about 8 to 12 seconds lead on my AI leading time. This was enough to win the stage overall by more than 1:00.