This combo is a great escape from the summer heat. Lots of singletrack intermixed with doubletrack sections. The Buck section has a few steep climbs, but nothing too long. The Aspen Loop has some great views.
Start with the Buck trail, parking at the north trailhead. Cross Hwy 90 and continue as if you were doing Buck Spur. When you hit the Dry Creek connector trailhead right. Cross Divide Road and find the singletrack just to your left. Follow this until you meet up with the Aspen Loop.
Take a right and enjoy the nice aspen groves and views this trail affords. There are a few intersections, but in general just stay left until you complete the loop. You'll pass a turnoff for the Little Red Trail and an unmarked right that leads to a small lake. Just after these, you'll be back where you started on the Aspen Loop. Head right again, cross Divide Road, and head right on the Buck Spur/Dry Creek trail.
Keep heading down the singletrack/loose doubletrack until you reach a small stream crossing. After this, you'll climb up for a half mile or so, pass a lake, keep going on the DT up and right before the DT crests out, take a right on the So Buck Trail
. This AWESOME singletrack takes you back to Hwy 90 and your vehicle.