For quick and easy Head-To-Head races head to the Airport. Drive up and down the Runway till you get behind somebody and initiate a race. Due to the way the Airport and the surrounding roads are laid out, how the Drivatar AI behaves, and the lack of checkpoints in Head-To-Head races your opponent will always stick to the roads while you can easily cut through all of the open space and head straight to the Finish line with no regard for the actual course layout.
Good Luck!