I wasn't even looking for this but I found it.

At the boarder between Grande Pastura and Montana there is a missing road with a collectable part at the bottom. At the edge of that chazm just off to the side of the road there is a soap box car just sitting there. I walked up to it and low and behold, I could get in. I rolled it back a little bit and went for a run with it. And suddenly a new challenge opened up called "Endless Runner". Looks like not many people have done this challenge. I went on for a bit, it is a lot of fun, made some jumps, got the main road and that was pretty much it for me and my go at it. I got out and I got the "Feat Festish" achievement. You get the achievement for doing all the feats in the game, the rest are rather easy to do by just playing the game, this one I guess is the only one that is hard to find.

Hope this helps!