A race Section is a part of the the race track. You know you've complete a section when you get a split time on the bottom of your screen. When you finish the race, the 2nd place opponent must be a full race section behind you, so at the part where you receive a split time before the finish. This is very easy further on in the game when you have a race with many classes. You need the fastest car for that class, with tuning done to it. You should be able to do it no problem.
Note: Can be done online.