The singletrack is closed closed May 1 - June 15.
Working your way up Red Sandstone Rd isn't great - there can be a lot of cars on weekends, but you get a surprisingly long and fun singletrack for all that work. Cool views of the ski area, but you definitely won't feel like you're off in the wilderness.
Starting at the Spraddle Creek Trailhead near the main Vail exit, grind your way up Red Sandstone Rd. The grade is moderate, but it's over 1,500 vertical to gain, so take your time. On weekends, cars can be annoying, but the road is wide and fairly safe for them to pass.
After a couple miles, you come to a fork in the road. Red Sandstone goes straight (the left option) and you would follow it to get to Bufferhr Creek trail or to the Piney Lake area. For this ride, you take the right option, which is called Lost Lake Road.
Stay on that road for only a couple hundred feet, and look for the singletrack (signed) heading off to the right.
The trail is a pleasant climb for 200 more vertical feet, and then the descent starts. The forest in this area is partially dead (pine bark beetles), but most of it is spectacular aspen groves. Smooth singletrack and great views abound.
After a couple miles of descending, you reach the North Trail (East). Take a right to get back to Red Sandstone Road and a short descent to where you started (shown here). Alternatively, take a left on the North trail for a longer option that includes more singletrack and a longer descent. This will spit you out right at the main Vail highway exit. Take the frontage road a couple tenths of a mile back to your car.