Bull Run gets high-speed downhill traffic, and is marked as a one-way downhill only trail.
Bull Run starts at the Mag 7 trailhead at the upper end of Gemini Bridges Road. From the right side of the parking area the trail jumps right into a mix of singletrack and slickrock with a small canyon to your left.
The canyon rim transitions from your left side to your right side about a third of the way into the trail. The canyon you'll see now is much larger than the previous one.
After a good bit of trail, there's a section of road to navigate and then lots more rolling and ledgy slickrock to cover. The views into the canyon are spectacular now.
After the second road crossing, the slickrock continues before a longer stretch of sandy singletrack. The trail will come to the Gemini Bridges Road where a new exstention of singletrack continues on the other side to quickly arrive at the intersection with Great Escape, Getaway and Arth's Corner.