You need a plenty of time to make this achievement :D all single races = all normal and mirrored tracks = 150 races.
So 30 races with 1st class, 30 with 2nd, 30 with 3rd, 30 with 4th and 30 with 5th. The biggest pain are 1-3 class because those cars are really slow... You can always check your status at Single Player races. If you got 2 stars from the left, that means you did it without nitrous ( the last 3rd star is for no crash victory )
Just race without using nitrous and win :D it's not hard to beat computer on single races. So just be patient and good luck wasting your time :)
( Oh and i found this achievement a glitchy one :( , i made all 150 races then my achievement didn't appear... so what i did? just removed my saves from the game and i did all 150 races again... ) Hope nobody else will get same :x