Joe - that puts it in perspective. Since most CONUS trail hiking is fairly easy (assuming basic fitness), I think that what you mean by challenging is that it would be a fairly long distance without resupply.
"I prefer to actually complete something (either loop or thru-hike)" You might consider dropping that idea. In Europe there are dozens, maybe hundreds, of named trails of 75-150 miles, so it makes sense to think about completing something. But in the US, the vast majority of wilderness areas have a network of trail segments that are not organized into a single named trail. Since many people like "named trails", those tend to get all the attention and are therefore more crowded -- John Muir Trail, High Sierra Trail, Colorado Trail, Wonderland Trail, etc.
Of the alpine areas, I'm most familiar with the Sierra range. The best part of the range is between Mono Recesses (Italy Pass area) and Cottonwood Pass. Just put together any loop, or one way hike (by hitching to/from highway 395 on the east side), and you'll be a happy hiker. Optimize your route to spend as much time above 10,000 feet as you can. Don't hesitate to plan a trail distance that's shorter than your total trip and then spend some time exploring off-trail. Going just 1/2 mile off trail gives you access to a lifetime supply of beautiful places. If you decide to go to the Sierra, then you need three things to plan your trip: RJ Secor's "The High Sierra: Peaks, Passes, and Trails", and two Trail's Illustrated overview maps - #205 and #809. Or, if you really want a single "just go here" answer, then the Big SEKI Loop would be an option.
Obviously, the Rockies (MT, WY, CO) has plenty of good hiking at half the driving distance from KC (12 hours instead of 24 hours). I can't help with specific recommendations there.