You mention something with 7-8 hours of Omaha - well, I can suggest
three very nice state parks in southerm Minnesota.  They won't fit
what you're seeking, but they are very nice, with 10+ miles of trails
each, and interesting features, and I'll plug them in the event you're
ever nearby.

Whitewater SP - 25 miles east of Rochester.  Bluffs and river valley.
Nice rocky points and overlooks.  Nice wildlife, too - I've seen deer
every time I've been there.  Trout fishing, too.
http://www.dnr.state.mn.us/state_parks/whitewater/index.html

Frontenac SP - on the Mississippi about 70 miles southeast of St.
Paul.  Nice trails below the bluffs about above Lake Pepin, a natural
reservoir.
http://www.dnr.state.mn.us/state_parks/frontenac/index.html

Blue Mounds SP - Maybe three hours north of Omaha, in extreme SW MN.
A rock wall rises out of the great plains.  There is a herd of bison
there (wild, but in an enclosure).  And a ancient rock wall ~1000 feet
long that aligns with the rising and setting sun on the solstices.
http://www.dnr.state.mn.us/state_parks/blue_mounds/index.html

There *is* one state park that's a hard 8 hour drive from you - Jay
Cooke SP just south of Duluth.  The St. Louis River flows through it
over exposed bedrock.  There are 50 miles of trails, including some
walk-in sites that are a good 4 miles from the nearest trailhead.
http://www.dnr.state.mn.us/state_parks/jay_cooke/index.html

Another hour up the shore you begin running into very nice state parks
every 15 miles or so overlooking Lake Superior and featuring rugged
trails and some walk-in campsites.

And an hour east of Jay Cooke SP, there's the Apostle Islands -
probably a hard 9-hour drive from Omaha.  But you'll find nice, large
islands out in Lake Superior that can be accessed by local boats.
Several of the islands could take a few days to explore.  And the
islands are wilderness - no vehicles, just the occasional private
property holdings that remain from before it was part of the Park
Service.
http://www.nps.gov/apis/index.htm

Erik