Start walking. Keep walking. What's the longest path you can take?
It starts in West Africa, on Liberia's Atlantic coast. Walk straight. You'll hit water again in 8,400 miles later, on the bank of the East China Sea.
Reminder: This is not a real trail people take— it's just the path you would walk, if you walked straight for as long as possible. Unlike the similarly long — but nowhere near as straight — Great Trail in Canada (which also has the advantage of being an actual trail), this "path" crosses some exceptionally dangerous territory — including but not limited to Libya, Iraq, Niger, and a bunch of other countries with extant State Department warnings. Plus live-fire zones. Plus an awful lot of China.
In a world, though, when it seems like all adventures have already been done, this would be one incredible experience.