United States
- Appalachian Trail - 1937 (3,500 km)
- Pacific Crest Trail - 1968 (4,270 km)

National Parks included in the PCT:
Kings Canyon National Park, Yosemite National Park, Lassen Volcanic National Park, Crater Lake National Park, Mount Rainier National Park, and North Cascades National Park

"The Pacific Crest Trail was first proposed around 1932 by Clinton C. Clarke as a trail running from Mexico to Canada along the crest of the mountains in California, Oregon, and Washington. The original proposal was to link the John Muir Trail, the Tahoe–Yosemite Trail (both in California), the Skyline Trail (in Oregon) and the Cascade Crest Trail (in Washington). The Pacific Crest Trail System Conference was formed by Clarke to both plan the trail and to lobby the federal government to protect the trail. The conference was founded by Clarke, the Boy Scouts, the YMCA, and Ansel Adams (amongst others). From 1935 through 1938, YMCA groups explored the 2,000 miles of potential trail and planned a route, which has been closely followed by the modern PCT route." - Wikipedia

New Zealand
- Te Araroa - 2011 (3,000 km)

Korea
- Baekdudaegan - 2006 (1,625 km) *735 km in S. Korea

South Korean National Parks included in the Baekdudaegan:
Jirisan National Park, Deogyusan National Park, Songnisan National Park, Woraksan National Park, Sobaeksan National Park, Taebaeksan National Park, Odaesan National Park, Seoraksan National Park

"The Baekdu-daegan is important in traditional Korean geography and thought, a key aspect of Pungsujiri philosophy and practices. It is often referred to as the "spine" or "backbone" of the Korean Peninsula, and depicted in various historic and modern artworks including national maps. Under traditional Korean thought influenced by Daoism and Neo-Confucianism, Mt. Baekdu-san is regarded as the northern root-origin of the mountain-system, and conceived-of as the grand patriarch of all Korean mountains; while Jiri-san at the southern end is conceived-of as the grand matriarch of all Korean mountains. - Wikipedia

Historic Sites:
Hwaeomsa (Gurye)
Jeoksang Fortress (Muju)
Seongju Royal Placenta Chambers (Seongju)
Mungyeongsaejae Pass (Mungyeong)
Mireukdaewon Stone Temple Site (Chungju)
Cheonjedan Altar (Taebaek)
Woljeongsa (Pyeongchang)
Odaesan National Historic Archives (Pyeongchang)

Documentary about the Baekdudaegan:
The Baekdudaegan Cultural Heritage Part.1
The Baekdudaegan Cultural Heritage Part.2

Thoughts about the potential or lack thereof to consider National Trails as world heritage sites? Pilgrimage Routes, Trade Routes, and pre-modern road systems are part of the world heritage list already. This is a very short list of national trails I am more familiar with (especially the PCT and Baekdudaegan). One question that would require some consideration: Would nominations be natural, mixed, or cultural landscapes? Are there any examples on the tentative list?

UK
- Hadrian's Wall Path - 135 km
Following the historic UNESCO World Heritage Site of Hadrian's Wall from coast to coast, the Hadrian's Wall Path National Trail is an 84-mile (135 km) pathway passing through a diverse landscape where magnificent moorland gives way to rolling fields and there are dozens of sights to behold.
The Trail is 84 mile (135 km) long. If you are an experienced walker then we generally recommend allowing 6 or 7 days to complete the whole Trail, although you might want to allow some extra time to visit some of the Roman sites that you pass. They all have museums with interpretive displays and they also provide refreshments.
+ there are small circular trails
https://www.nationaltrail.co.uk/en_GB/trails/hadrians-wall-path/circular-linear-walks/
Personally the thought of having UK's trails inscribed is great, especially at the Seven Sister's cliffs and the Cornwall/Devon area, but I think inscribing it would be hard. A lot of trails around the UK also seem to pass through private land and farmer's generally don't care about people passing through (very unlike it would be in the US!) as long as you don't mind stepping on cow pats or get harassed by sheep wanting your food :)

Korea also has that Seoul City Wall Trail but I don't know about National Trail status and the if it's fully hikeable. There were always sections off-limit at times but I thought it was all open now.

Seoul City Wall is 18.627 km and is now fully open. Bukhansanseong is 12.7 km long. With repairs to the connecting fortress between them "Tangchundae" ongoing, the whole capitol defense system will connect more than 30 kilometers of trails in the near future.

I think the John Muir Trail (340 km) would be a worthy extension, linking Sequoia and Kings Canyon National Park to Yosemite National Park. Since this trail was completed in 1938 and partially completed decades before the cultural value is significant. The trail has plenty of connections to the Early Park Service and Conservation Movement in the United States.

I suppose the question should be how might a "National Trail" demonstrate OUV? Is the primary value, its relation to engineering, the Conservation Movement, Early National Parks and/or the Civilian Conservation Corps (CCC)? Is the value more defined as a cultural landscape, as is perhaps the case in Korea's Baekdudaegan?

Also worth considering is the International Appalachian Trail.

Once it became known that fragments of the ancient mountain range exist in Canada (inc. Gros Morne), Greenland, Morocco , Scotland, N.Ireland (inc. Giant's Causeway), Wales, and some other parts of the Atlantic coast of Europe, work began on setting up a series of trails in all of these locations. This could eventually include a few more current WHSs, or be a nice transnational property in its own right.

This seems really stretched... more of an attempt to recall attention by means of a brand than anything else, as it clearly apperas from the article.

In general I do not see potential, or even the bases for OUV in these trails. There are various "linear" sites on the List, but they are normally substantiated by some extra significance and/or concrete manifestation or technical achievement. For example, the Incan trail was an effective communication route whose creation and maintenance was only possible by a great communal effort; the trans-Iranian railway is a work of engineering; the pilgrimage to Santiago de Compostela is more on the intangible side, but it is "focused" by so many devotional buildings on the way, and so on. While these other trails are more of a connection of dots: sometimes they require some maintenance and planification, too, but in my opinion they are just "abstract superstructure". By the way, I'm also strongly skeptical of any value associated to the via Francigena, for example, and Santiago de Compostela is also more on the dubious site to me.

So, very interesting routes for wonderful and sometimes holistic experiences, and so excellent connections, but not even "listable objects" in my opinion! :-)