I debated long and hard about the rating for this trail. Technically, it should only be beginner/intermediate, but there are small parts that can be considered intermediate. Most people, I think, would fall somewhere between the two.
From the end of N Arroyo Blvd, take Fern Truck Trail to the right, and follow the sign to Brown Mtn. In a few hundred yards, splash through the first, and deepest, creek crossing of the day and climb the baby-head rock garden to another trail split. To the right is the bottom of El Prieto, the entire reason you're here. To the left is the first switchback of Fern Truck Trail. Go left. From there, it's 2.6 miles to the top of Saddle Junction, and over that span you'll climb 1100 feet, a steady 8% grade.
There are no trail splits, so just put your head down and find a rhythm, and soon you'll reach a plateau beneath a tight group of tall pines, the only ones around. This is Saddle Jucntion. Uphill and to the left/north is a three-mile out-and-back spur to Brown Mtn. Downhill and straight/east is Fern Truck Trail for another mile to the top of El Prieto Trail
. This last downhill section is actually pretty fun with some whoops, some cliff exposure, a small water crossing, and some fast, smooth downhill.