To my knowledge, here are the longest linear trails we have. Sources for these trails vary, and many do not have an official internet presence.
- The Lone Star Hiking Trail is currently measured at 128 miles, including loop trails. It begins in the west near the small community of Richards and ends in the east near Cleveland. It passes through the Sam Houston National Forest as well as a small portion of the Big Thicket. A portion of the trail slices through a corner of Huntsville State Park outside of Huntsville.
- The Northeast Texas Trail is working to become the longest trail in Texas. It follows an old rail bed from Farmersville through 7 counties to New Boston (near Texarkana). I'm not sure of the length, but highway miles from Farmersville to New Boston totals 123 miles.
- Caprock Canyons Trailway, a 64 mile long trailway on a former railroad bed, is located in the canyon country of West Texas.
- Trail Between the Lakes is located in deep East Texas, between Sam Rayburn Reservoir and Toledo Bend Reservoir. Total length is 28 miles, all in Sabine County.
- Four Cs Hiking Trail lies within David Crockett National Forest, roughly between the towns of Alto and Ratcliff. The linear trail is about 20 miles. The unusual name comes from the Central Coal and Coke Company, which logged timber here in the old days. There is even an old rail bed that the trail follows at times. Donna and I actually hiked part of this trail years ago. You can view my hiking report here.
- Lake Mineral Wells State Park Trailway stretches about 20 miles from Mineral Wells in the west to Weatherford in the east. Access points are available in both towns named above as well as from within Lake Mineral Wells State Park and the small community of Garner. This trail is part of the Rails to Trails project. This is another trail that Donna and I have partially walked. We started at Weatherford, went west for 5 miles, then returned for a total of 10 miles. I never wrote a review of that hike.
The above are not all the trails, of course, but are only the ones I'm familiar with to some degree. The state and national parks around Texas as well as national forests have miles and miles of trails, but nothing of length that extends from point A to point B. For example, San Angelo State Park near me has over 50 miles of trails, but within that 50 miles are a dozen or more trails of varying length.
One of the best sources for hiking in Texas is a website called Texas Hiking. In the past, I had an account on this site and contributed regularly. I have not used the site in years, though. Some of my posts are still available under the user name "kcameron". You can also visit your local library and search for hiking books by Laurence Parent and Mickey Little. Both authors have written extensively about hiking and camping, in Texas as well as other states.