Buckeye Trail-1,444 miles and longest circular trail in the USA with it being the third longest circular trail in the world. Information can be found at Buckeye Trail.
Ohio to Erie Trail-will be fully complete and connected in 2015. 85% complete now and many people have already hiked/biked it as it is open. Not much information on the Ohio to Erie Trail sadly. Information can be found at Ohio to Erie Trail. Starts in Cincinnati and ends at Lake Erie.
Te Araroa Trail-this is 3,000 KM or 1,864 miles long. Stretches the entire length of both the North and South Islands. Takes 3-6 months to complete. Information can be found at Te Araroa. Starts in Cape Regina and ends in Bluff.
Sadly I do not think I will ever get over my fear of heights and the Appalachian Trail is crossing mountains and valleys pretty much the entire way. So that is out but if you want information the Appalachian Trail or AT for short is 2,180 miles. Can be taken Northbound from Springer Mountain, GA to Katahdin, ME or from Southbound from Katahdin, ME to Springer Mountain, GA. Only 1 in 4 people who go out on the trail ever finish it. Goes through 14 states. Information can be found at Appalachian Trail Conservatory. After looking at the estimated level for each section, sections ME and NH are 6-10 and 10 is not recommended for short people or people with a fear of heights.
Here is a neat trail I found a few days ago: Sultans Trail. Goes through seven countries. Starts in Vienna and ends in Istanbul. Suleyman Kanuni, sultan of the Ottoman Empire, made the trip from Istanbul to Vienna for the first time in 1529 and again in 1532. Both times he didn't succeed in penetrating the city of Vienna. Known as the 'path of peace'. Information can be found at Sultans Trail. I want to try this trail.