The Appalachian Trail is one of the longest continuously marked footpaths in the world, measuring roughly 2,180 miles in length.
Need a reason to hike the Appalachian Trail? There are plenty. Sure, it may not be the longest hiking trail in the world anymore, but it still goes through 14 states, six national parks and eight national forests.
Appalachian Trail - Wikitravel
Appalachian Trail Map. I think this one is better at encompassing the magnitude of such a hike.
10 Things You Need to Hike the Appalachian Trail- Infographic
10 Things You Need to Hike the Appalachian Trail. (Great info even if you're not doing that one!)
Smoky Mountains National Park. From the top of this you can see 4 different states. Long walk but worth it!