After passing the first vision of the woman/second note, you'll go down a hill until you see a deer on the right side of the road. Move out to the left side of the road, away from the deer, and towards a large rock formation that's visible a little ways off. To the left of the rock formation are three graves. In the middle of them are a bunch of flying lights/wisps. Just walk straight into them, and you'll get a speed boost for a short amount of time. The achievement will pop.