A 110 mile rail trail.  https://gfp.sd.gov/parks/detail/george-s--mickelson-trail/

The Lean Horse 100 covers most of this trail: https://leanhorse100.com/

Fisher Peak is the highest summit in the Southern Rocky Mountains at 2846m. Its prominence and dramatic shape make it easily identifiable from the surrounding communities.

The hike is a steep and sustained out-and-back with a brief respite near the upper cirque. The last 300m of elevation are gained by steep scrambling from the south col to the summit, where there's a stunning 360 degree mountain view. 

The ascent has a Strava segment:  https://www.strava.com/segments/10266617

The goal is to summit the tallest 50 peaks in Montana using the international standard of ~100m of prominence for unique peak criteria.

See below list for peak names.

Total distance is ~200 miles, nearly all off trail, and elevation gain is going to be between 100,000-150,000 feet. 

Peak List (by elevation):

1- Granite Peak, 12,799

2 - Granite NW, 12,745

3 - Mount Wood, 12,660

4 - Castle Mountain, 12,612

5 - Whitetail Peak, 12,551

6 - Silver Run Peak, 12,542

7 - Rainbow Peak, 12,540

8 - Tempest Mountain, 12,469