Rusty Spur has a main loop that's 1.3 miles of dirt plus 1/10 mile of paved trail that closes the loop. There's a 0.3 mile ST connector for those who are heading southbound to the loop from the Brand Trails parking. (This little connector isn't exciting, and you won't miss much if you do only the main loop.)

The shortest ride is a lariat loop from the lower Brand Trail (Bar M) parking. The ride is 4.2 miles total, with only trivial elevation change. I suggest you pop through the fence and ride paved Old 191 southbound, rather than taking the chunky gravel Bar M. (The Bar M and Old 191 run alongside each other in this area, separated by a fence with periodic walk-throughs.)

You can also pedal directly to the trail from Moab by following the paved Old 191 bike path. From the Colorado River, it will be 6.5 miles to the loop, with 650 feet of elevation gain.

All three trail entrances from Old 191 are marked with signs.

At first, the loop will look like ultimate boredom. As you look west from the trailhead on Old 191, you'll see grass and a highway. Have faith. I wouldn't send you onto a trail that pointless.

As the trail turns south, you'll quickly enter a broad ridge studded with junipers. To the west are dark brown slopes of Moenkopi mudstone, topped with cliffs of Wingate sandstone. The trail begins to meander and behave more like a mountain bike path. But the riding is still easy and non-threatening.

To the south, you're looking down the path of the Moab Fault, where Highway 191 descends toward Moab and Arches National Park. As you reach the southern end, there's a nice view of the paved bike path as it climbs up from Moab.

Getting there:
From the north: Drive south on US-191 for 22 miles south of I-70, then follow the directions below.
From Moab, drive 7 miles north from the Colorado River on 191. When you see the Gemini Bridges parking area, look for a gravel road on the opposite side (east, away from the cliffs). Turn onto the gravel road and immediately go right. The first parking area is reserved for the Bar M diner. (Paid parking is available at the far end of this lot GPS N 38° 39.38' W 109° 40.66'.) Continue driving south until you approach the fence (and paved bike path) on the right, then turn 90 degrees left (east) and drive to the gravel parking area in the little valley. The ride will start by backtracking up to the kiosk at the paved trail.
Alternate: From Moab, ride the Old 191 paved trail until you reach the Bar M kiosk. (You'll pass the south entry to the loop about 1/10 mile from the kiosk.) Across from the kiosk -- to the west -- is the singletrack.