Just wanted to add my own helpful tips to anyone wanted to do this achievement.
I currently have 2 supersport wins and 2 superbike wins, therefore the mixing and matching of wins doesn't work for the achievement, so it needs to be 3 Supersport wins OR 3 Superbike wins.
The AI times I find are so random, I had a faultless 1hr 2sec run (with dashboarding and restarting if I did crash) and 1 of the AI would keep beating me so I restarted. I've found out that if you finish the race and if you don't win, you can dashboard out before returning to the main menu, restart career and you'll be at the last section of the race. The AI will have totally different times, and a few attempts I won the race, saving me do a full hour again!
Hope this helps