It's obvious that I enjoy hiking. I've always enjoyed walking in the woods. I've probably hiked at least some portion of the best hiking trails in Texas with 1 or 2 exceptions (the Lone Star Trail and the Trail Between the Lakes come to mind). But I really regret that I've never hiked any portion of the big 3 trails in the continental U.S.
The Appalachian Trail is the oldest of the 3 big trails. Almost 2,200 miles long, it passes through 14 states from Maine to Georgia. Although the idea for the trail was conceived about 1920, it was not really completed until 1937. At the time, the idea was to provide a trail that people could hop on and off for a few days of hiking. It was inconceivable to the early planners that anyone would ever hike the trail from start to finish, but Earl Shaffer did just that in 1951. Today, somewhere between 2 and 3 million people hike at least a portion of the trail each year, and more than 500 or so complete a "thru-hike" each year (an uninterrupted hike from start to finish). It's a rigorous undertaking that requires from 4 to 6 months for most people.
The Appalachian Trail is no longer the longest trail in the country. It has been surpassed in length by both the Pacific Crest Trail and the Continental Divide Trail.
The Pacific Crest Trail stretches about 2,650 miles from Canada to Mexico, passing through only 3 states: Washington, Oregon, and California. The Continental Divide Trail is the longest of the 3 big trails, measuring about 3,100 miles. It's route traverses Montana, Idaho, Wyoming, Colorado, and New Mexico.
Although I've never hiked any of these trails, I've read a number of stories of people who have. From my reading, I believe it is probably much more difficult to hike the two western trails than the Appalachian Trail. The Appalachian Trail passes through more populated areas, sometimes even passing through towns or very near towns. This is convenient for resupplying, resting up in a hotel, and eating at local restaurants. It is more difficult to resupply on the western trails, thus requiring much more planning and often coordination with others. The western trails also encounter larger elevation differences. The Appalachian Trail offers its own challenges, though, such as unpredictable weather conditions in the White Mountains of New England.