I did some mtbing years ago in Nebraska National Forest, and as I recall the trails were all very sandy. I expect similar in Chadron. Bring some big fat tires if you go!

To anyone interested,

Finished this route with some deviations.

Some Stats:
All approximate, I am still away from my computer and have not processed any gps info.

262 miles
33k ascent
Days riding 6 (about 12hrs a day)
Max miles/day 58
Min miles /day - 20 (this was brutal)
supplies purchased on route - 1 meal (hamburger and fried mozzarella), 1 beer.
Miles covered with rear brake only - 200ish.
Moose seen - 1 Bull

Overall this was a pretty awesome adventure.   On day three I spent most of the day pushing, pulling, and dragging my bike through heavy timber, massive blowdown, and over extensive boulder/scree fields following a track/trail that was non existent. I think I had ride(ha!) time of about 12 hrs and covered about 20ish miles.  This day was practically criminal - I basically manhandled my bike down a 2500 ft descent .

This is beautiful country with many options.  I could make it better now that I know a bit more.  I had to shorten the intended route due to time constraints.  Days 2 and 3 were really slow. I will try and write a better review with photos when I get home.  Cheers all!

I'd love to see pics and/or gps info, and any tips you have for next time. My parents moved to Sheridan a few years ago and I'd really like to put together a trip up there next summer.