This is literally as it sounds. You just have to win one race, at 100% race length. I did it in the Street Stocks at Sterling County Speedway. To do this, from the main menu, select "Quick Race," and then select from one of the 3 car types. Scroll through and pick a car you like. Then the track menu should be your next screen. Look for a track that looks fun or interesting for you (you will be turning a decent number of laps). Before using
to select your track, hit
, and the race settings menu should come up. Go to the second to last setting on the list "Race Length," and put the slider all the way to the right. This is the 100% setting. You can also run this race on whatever difficulty or settings you'd like, but you must be on the 100% race length. Back out of the menu and select your race. Win all the heats and the A-main, and the cheevo should pop when you get back to the track selection screen.
Completing the race will also unlock:
And if this is your first win of the whole game, you will also unlock: