The Church Rocks Loop is a popular trail just north of Washington (about 10 miles north of St. George). The loop itself is 3.1 miles of 80% slickrock. It's a fairly rapid ride, with little elevation change (about 150 feet), so you may want to do the loop section a couple of times. Because of some steep and rough spots, Church Rocks ranks upper-intermediate overall in skill level, but less-skilled riders manage by walking the tricky spots.
Your overall ride will be longer. Church Rocks doesn't have its own trailhead, so you'll have to plan to ride there via Prospector, Grapevine, or The Tunnel. Most riders now connect to Church Rocks via the Prospector Trail through the Cottonwood Trailhead. More on that later.