Wasatch 100
Not really the toughest, but close.
The 2010 Wasatch Front 100 mile Endurance Run starts at 5:00 AM sharp on Friday September 10, 2010. The race begins just past the entrance to the East Mountain Wilderness Park about 17 miles north of Salt Lake City.   The Wasatch 100 is a point-to-point race that traverses the heart of the central Wasatch Mountains, one of the most beautiful ranges of the Rocky Mountains. The course begins in Kaysville, running north to the Bonnevile Shoreline Trail to Fernwood Picnic Grounds, the foot of Frnacis Peak, and ascends nearly 5,000 feet in 5 miles to the ridge line. The trail then turns south and follows the crest of the Wasatch along Francis Peak Ridge, through Farmington Flats and Arthur's Fork, along Sessions Ridge, over City Creek Pass, Big Mountain Pass and Bald Mountain, through Parley's, Lamb's, and Millcreek Canyons, by Desolation Lake and along the Wasatch Crest trail, through Big Cottonwood and American Fork Canyons, and up to Pole Line Pass.  After traversing along Mill Canyon Peak on the western side, the trail turns back north on its eastern side, down Pot Hollow Canyon, up to   the Crest Road in Wasatch Mountain State Park, then drops down Lime Canyon slipping over to a snowshoe trail just before coming out on the Snake Creek road and the run to the finish at The Homestead in Midway, Utah. 

If you have ever stood in the Salt Lake City airport and looked out at the spectacular range of mountains to the east that rise high above Salt Lake City, that is the Wasatch Range.  You are also looking at the ridge line the Wasatch follows for about 80 miles.
  The run starts north of SLC, and heads north for a few miles.  It then turns east and climbs straight up to the crest of the ridge line.  The run follows the ridge crest for about 40 miles until dropping down to Interstate 80.  There it crosses under the highway and heads back up, crossing above The Canyons and Park City ski resorts, (just above life 9990and Peak 5 Lift at the Canyons), before heading down to Brighton ski resort in the Big Cottonwood Canyon.  It then climbs out of the canyon to the ridge crest and continues to the ridge line just above Alta Ski Resort.The run continues along the ridge crest for about 15 more miles before dropping down in the town of Midway, Utah.  The Wasatch gains 26,882 ft and looses 26,131ft over the 100 mile race.  (Hardrock gains and looses 33,100 ft at an average elevation of over 11,000ft.  Wasatch never even reaches 11,000 ft.)  Definitely not the hardest, but plenty tough!  Actually it is probably the second hardest run on earth.  Most ultrarunners agree that Leadville is the third hardest.  Although Leadville climbs substantially less than either Hardrock or Wasatch, the average elevation is higher than the highest point of Wasatch.
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This is a link to a really cool flyover of the Wasatch 100 course prepared by TheRunScout.com:  Flyover

This is a map of the entire Wasatch 100 course.  Click on the picture to view the Larger Image or click the following link to view it on Goggle Earth.  Google Earth View.  Follow the instructions in the first paragraph.
The Run for Kids Challenge
12 Hour, 50K, 5K
May 28, 2011