This will be a breeze if you're aiming to complete the game 100% (just don't exclusively use fast travel).
If you manage not complete this before you complete all the cases and just want to go back and finish up everything else. To do this quickly you'll want to grab the Duesenberg Walker Coupe which you can find up at the Automobile Club of Southern California building complex at the corner of Santa Monica Boulevard and El Centro Avenue. This video will also show you where it is. Once in posession of the car, go to the Nash Army base in the south-east part of the map shown in the picture.Go to any long street, and drive up and down. Drive through the wooden gates and onto the track. Drive around the track but be wary of turning corners at full speed as there are plent of trees to hit if you swerve off the track. In the middle there is also a bridge which can act as a ramp if you get bored driving off into the sunset.
The length which needs to be driven is an allusion to the year in which the game takes place.