If you have a car that you're trying to get up to 50 miles driven for the One Careful Owner trophy, it might make sense to use that car for this trophy.
I started at the southwest corner of the map, and traveled east. I tried to follow the solid yellow route around the map. It's ok if you press the back button to look at your map while you are doing this - it won't count against you.
I did not use the speed limiter, but kept an eye on my speedometer and tried to keep it between 40 and 45, just to be safe. I found that pressing R2 every half second or so maintained the speed I wanted in the car I was driving.
The trophy unlocked when I made it around near the Hill of Tara in the northwest corner of the map.
Good luck!