Personally, I found the best place to do this was in the canyon at the edge of the Ashlands leading to the Iron Canopy - ya know, the area right after the Soul Bridge. Sure, you could just run Ruin around in the Stygian arena forever, but the scenery sucks, and it makes a bland, grinding Achievement even more mundane. The canyon is a long run (approx. .7 miles from the Ashlands end of the bridge to edge of the cliff), devoid of enemies (except for a randomly appearing horseman, who is easy to avoid), and the Soul Bridge offers about .18 miles per lap of full charge speed without burning a boost. As an added bonus, every time you return to the Soul Bridge the game Auto-Saves your progress, assuring you no lost mileage or time! Sweet!
Simply charge across the bridge, run the length of the canyon - boosting as you go - then swing around at the edge of the Iron Canopy cliff/entrance, and race your way back and across the bridge again. I've clocked it at just about 45 seconds per length, or about 1 minute 30 seconds per lap, logging you 1.4 - 1.5 miles per lap.
Yeah, it still sucks, and it is a grind, but I found this method far less mundane than simply running in circles, and it might actually be a little faster, given the free boost speed across the Soul Bridge. :)
Hope this helps! PROST!