The 109 mile Mickelson Trail starts in Deadwood. The Deadwood Trailhead is just off Sherman Street. This trail is open to hiking, horseback riding and biking. Near by, the Centennial trail is 111 miles long and goes right through the middle of the Black Hills National Forest.
Fresh water is not always found at trailheads in the Black Hills, be sure to pack more that you think you’ll need. Snacks are a must, I always seem to hike further than I anticipate. The weather changes quickly, include appropriate clothing in your pack and be prepared for an afternoon thunderstorm.
If you want to read more about hiking in the Black Hills I recommend Exploring the Black Hills and Badlands: A Guide for Hikers, Cross-Country Skiers, & Mountain Bikers and Best Easy Day Hikes Black Hills Country (Best Easy Day Hikes Series)