Two-two rides in one! The Centennial Trail starting at Alkali Creek is one of my favorite places on Earth. The trailhead is in the valley where the Black Hills meet the prairie. From there, it goes up into The Hills for a couple thousand feet. A lot of folks just ride up Bulldog and back down, but going past Bulldog another 5-10 miles is the best part of the trail! Yes, it's a hell of a climb.
It's like two rides. 1 1/2 hours of climbing, then you turn around and point back downhill. From the high mountain meadow it's an absolute shred-fest for a good 30 minutes straight! Ground missile. Hang on, keep your tongue inside your mouth, and the tears out of your eyes.