The ultimate long trek in the world, I reckon, is the Nepal section of the GHT.
The Great Himalaya Trail is one of the longest and highest walking trails in the world. Winding beneath the world’s highest peaks and visiting some of the most remote communities on earth, it passes through lush green valleys, arid high plateaus and incredible landscapes. Nepal’s GHT has 10 sections comprising a network of upper and lower routes, each offering you something different, be it adventure and exploration, authentic cultural experiences, or simply spectacular Himalayan nature. …
read more on The Great Himalaya Trail
2011 guidebook – Nepal Trekking & the Great Himalaya Trail (Trailblazer)
Thank Richard for the update. And for the link to an interview with thru hiker Shawn Forry (trail name Pepper). He and Trauma did 57 days carrying their own gear cache to cache. Some harsh sections.
One big problem with Nepal is the complexity and cost of trekking permits. They tried to do it legal, relying on Adventure Alliance in Kathmandu to organize the paperwork.