Jason Wish submitted the route:

The Burr Oak Backpack Trail (Burr Oak State Park) is a scenic journey up, over and along the wooded hills and ridges that define the shape of the Burr Oak Lake’s shoreline. For the majority of the trip, the trail is not far from water’s edge. Some stretches of the trail are near lake level, but most are anywhere from a few feet to over 100 feet in elevation above the lake. Ascents are generally short and mild with only a few exceptions, making this a nice trip for beginners.

At one time, the rumble of freight cars reverberated through the narrow tunnel. Wooden trestles, with their system of slanted supports and horizontal cross pieces, spanned numerous bluffs and creeks. Passengers in Pullman cars clocked the miles with these and other landmarks as they traveled one of southern Illinois' most scenic routes.

This course, a bit outside of Harrisburg, seems to have been a "thing" since at least the late 1990s.  That said, we have little info on past FKTs.

A mix of fire road, single track, rail trail, Horse-Shoe Trail, Appalachian Trail, roots, rocks, leaves, dirt and mud. Start finish is at the same location.

Submitted by Dana Wood:

Founded in 2016, the Lake George 12ster provides a formidable taste of what the Adirondacks have to offer.
Traverse 12 peaks, nearly 40 miles of trails, and up to 9,000 feet of elevation gain
Become a 12ster at your own pace, or tackle them all in a 24-Hour Ultra
Winter and Ultra Winter categories available for those that love to embrace the cold

Vehicles may be used to shuttle between trailheads, or you can do the entire thing "self powered", utilizing a bike ("multisport") or not!