Need for Speed: Hot Pursuit

To go for this achievement the first thing that you should do is race the "Sunset Racers" race in the Porsche DLC. Whether you win or lose, this will earn you 19.5 miles (approximately) to your ultimate goal of 20.

Then, simply find another race where you can use this car, and FINISH that race to unlock the achievement. There are many other races where this car can be used, so I will leave the choice to personal preferences.

BUT NOTE: (With thanks to Hirsute Dave) "The "Barnstormer" gaunlet in the DLC is one where you're forced to drive the Speedster so it and "Sunset Racers" will unlock the achievement for you.

EDIT NUMBER 2: (With thanks to many people!) Driving "Sunset Racers" with a slightly bendy / wiggly course, including short cuts, you might just get your achievement in this one race alone.

Winning isn't a requirement but will help towards getting "The Porshe Principle achievement."

It should be noted that you HAVE to go for this achievement in races. Free roam will NOT count towards your total.

Instead of going out of your way to get this achievement, I got it by just going for the golds in the events.

One thing that should be pointed out that hasn't been pointed out yet is the cop version of the Speedster DOES count towards the achievement. When I was going through the Porsche Pack, I did two cop events in the Speedster and didn't get 20 miles in them. After I completed the cop events, I started doing the race events and once I finished Barnstormer once, the achievement unlocked for me, despite Barnstormer being only 8.4 miles long. In conclusion, you can use both the cop Speedster and the racer Speedster for this achievement at the same time and stack the values of the two to get this achievement instead of having to go out of your way to find a race long enough for you to get it in one run or doing runs over and over again.

Hope this helps!