So this will almost certainly be the last achievement you do. Definitely wait until you have all others until you go for this.
If, like me, it is your last achievement, you'll want to know the fastest way to do this. I decided to test every fast track to see which I could get the highest average speed on, and thus cover the most distance in the shortest amount of time. I tried all fast tracks in Chile, Australia, Portugal and of course Finland (you've probably seen the loading screen tip telling you that the last 9 of 10 Finnish rallies were the fastest that season, and earned the nickname "Finnish Grand Prix"
Finland's Ouninpohja Reverse course comes out the fastest track in the game. I was able to complete it's 10.35km in 4:45, giving an average speed of 130.74kph.
How to go about getting this achievement in the easiest way possible:
1. Make sure you have damage set to visual only.
2. Enter quick play > Finland > Ouninpohja Reverse.
3. I found the Hyundai to be the easiest car to control, and achieved all of my fastest times with it.
4. Tune your car! Where there are two values, the first is the front value, the second is the rear value.
Springs: 15,000 - 20,000
Shock compression: 2,500 - 2,500
Shock rebound: 1,000 - 1,000
Ground clearance: 30 - 30
Anti-roll bars: 20 - 18
Leave as is, move LSD preload (central) to 90. This will allow you to use your handbrake easier, as it will let your rear wheels spin at a different speed than your front. Helps you get round corners smoothly and in a controlled manner.
Brakes: 55. Means you can tap the brakes without the rear of the car sliding out much.
Gear ratio: 4.5
This set up will allow the car to be driven very quickly without it bouncing much. You will really have to flick it into the corners to get it to turn, but it means you'll maintain control in the high speed sections.
5. If you get a penalty, just keep going. If you crash awkwardly, just hold Y to reset. Penalties don't matter here.
6. At the end, just go to Restart (unless you want to save your ghost to race against in future attempts). Hitting restart still counts towards your total.
7. Don't be afraid to slam into the wooden railings - you don't take damage. Particularly useful for the first corner and the section with houses.
8. Take it easy. Try to enjoy this, you might be doing this for a while. Just focus on gently flowing through corners at a respectable speed (when you're not smashing into the railings) rather than going for a world record. Just keep the car chugging round the track.