I live near the Black Hills of South Dakota where the Mickelson Trail goes from Deadwood to Edgemont. Total of 114 miles with three tunnels, a short leg to Lead and one to Custer State Park.
The only place I use my mtn bikes, have ridden total length twice. Scenery is great, inclines are long, up to seven miles. That means declines are also long.
I have met many tourists that came just to ride this trail. Met a couple guys last year from Trek.
There are a couple organized rides every year, if you are near check it out.
Great ride with the grandsons!