Length: 4834 miles (northern route)
The following is from Wikipedia's American Discovery Trail entry:
The American Discovery Trail is a coast-to-coast hiking and biking trail across the mid-tier of the United States. It starts on the Delmarva Peninsula on the Atlantic Ocean and ends on the northern California coast on the Pacific Ocean, and is signed on over 6,800 miles (10,900 km) of trail. This includes the doubled trail stretches. It is possible to complete the coast to coast hike by covering just over 5,000 miles (8,000 km) of trail. It is the only non-motorized coast-to-coast trail.
The trail passes through 14 national parks and 16 national forests and utilizes sections of or connects to five National Scenic Trails, 10 National Historic Trails, and 23 National Recreation Trails. For part of its distance, it is coincident with the North Country Trail and the Buckeye Trail. From western Ohio to northern Colorado, the trail has two parallel stretches.