The oldest footpaths have probably evolved into highways by now and many highways (in the US) began as military highways. Many old roads have been bypassed by newer and straighter and wider highways. But I wonder about some new roads. You've probably all seen illustrations of how the old Roman roads were built, that look like they were two or three feet thick. These days, it seems like about six inches is good enough, at least until it needs repairing again.
Dedicated hiking trails around the country have various origins. The Appalachian Trail, which I am most familiar with, was apparently created from scratch sometime in the 1930s but it has changed a lot over the years. Originally, the route included some stretches on local roads.
When we visited the U.K. a few years ago, there was a functional canal that went by the little town Cropredy. We knew there were canals in England but for some reason they were not in our consciousness when we went. Anyway, there are a few canals here, too. Locally, there is the old C&O canal that goes west from Washington, D.C., to Cumberland, Maryland. Although only a few portions of the canal can still float a boat now, the canal was in use into the 1920s. However, the towpath is still intact practically all the way, I think, and it makes a really great place for a long hike, although I've never seen mention of anyone hiking the entire length, at least not all at once. It's almost 185 miles long. That would be a respectable long-distance hike but it's level the whole way and offers good views of the Potomac River, too. The possibilities for hiking nude are pretty limited, though, as is the parking anywhere near Washington. It mostly has a gravel surface and it is now a national park.