Hadrian’s Wall, built by the Romans in ancient England, was more political than practical, Rory Stewart writes. (De Agostini/Getty Images)

Hugh Thomson is the author of “The Green Road into the Trees: An Exploration of England.”

William Wordsworth was ever on the move, composing as he walked. That he chose to ramble — over his lifetime walking a staggering 175,000 miles — contributed not only to his happiness but to ours. “We are indebted for so much of what is most excellent in his writings” because of his penchant for walking, the essayist Thomas de Quincey once remarked.

In Wordsworth’s day, walking was the act of a radical; it was to ally yourself, as the young poet wanted to do, with the peasant and the peddler. While more aristocratic artists of the day might take the Grand Tour by coach to Italy, Wordsworth chose to walk through France during the year of its revolution. It helped him feel connected to the world and to people, to make an atlas of his feelings and spiritual progression.

Rory Stewart follows in that mold. For his first book, the acclaimed “The Places in Between,” Stewart walked across Afghanistan just weeks after the fall of the Taliban, an adventure that was both brave and revelatory. And this was just the beginning of a far longer walk that saw him cross Pakistan. Next, Stewart, a former soldier and diplomat, went to Iraq, where he was appointed a provincial governor after the 2003 invasion. It was an experience he wrote about memorably in “The Prince of the Marshes,” a sobering account of the fog of ignorance that pervaded the administration of that country.

Now he has come home, so to speak, to Wordsworth country. In “The Marches,” Stewart has written an account of a walk across and around England, beginning with a traverse along Hadrian’s Wall, built when a Roman emperor wanted to keep out migrants. The wall took 20,000 men more than a decade to build and required more stone and labor than the pyramids. As Stewart walks along what remains of the wall, he imagines it, “stone by stone, stretching fifteen feet high, entire and intact, from coast to coast, running straight up hillsides, down gullies and over cold rivers.”

However, and with obvious resonance for the wall that may one day divide Mexico from the United States, he is quick to point out that Hadrian built his more as political symbol than practical barricade. Indeed, archaeologists concur that it was far from effective, was “porous” and, after a while, may have been most useful as a way of employing otherwise idle troops in wall maintenance. It became “a zone of cultural exchange and encounter rather than a military barrier.”

The trope of the travel writer who returns to describe his own homeland as if it were another foreign country has become a familiar one — exemplified most recently by Paul Theroux, whose “Deep South” was a prescient account of the forces beginning to stir there. So it is natural that Stewart should find echoes along Hadrian’s Wall of his experiences in Afghanistan and Iraq. Britain sucked in Roman troops. It was considered an exceptionally difficult province to govern. The wall was a radical solution to a grave problem. The Romans struggled to hold Britain with 50,000 men — the proportional equivalent, Stewart says, of the United States and its allies keeping half a million troops in Afghanistan. “The problem was simply that the occupier lacked the knowledge, the legitimacy or the power to ever shape such a society in the way that it wished,” he writes. It’s a judgment he makes about both Britain then and Afghanistan and Iraq today.

Stewart remarks, too, on the way the Romans drove the wall across often unsuitable landscapes: “It looked like the straight lines drawn across flat ground by colonial officers in Africa.” Not that he is wholly concerned with the past. The area he walks through is now one of the most deprived in Britain, with high unemployment rates, and voted heavily for Brexit. As it is also near the Scottish border, and Stewart campaigned against Scottish independence and the division of the country, issues of national identity greatly concern him.

For much of the walk along Hadrian’s Wall, Stewart is accompanied by his 89-year-old father, an ex-service man who injects a bluff candor into the proceedings and is one of the book’s many strengths. With great affection and frankness, Stewart charts their relationship; the book could almost have been subtitled “A Walk Around My Father.” At one point, discussing the wall, the younger Stewart realizes that his father sees it not as a way of keeping the barbarians out — as most commentators do — but as a way, rather like the Berlin Wall, of keeping the Romans in. He had, after all, been a Cold War intelligence officer.

Stewart shows self-deprecating humor throughout, and his prose is always cool and lucid. But there is a lot of it. Just as I suspect that no reader has ever wished that Wordsworth had written more, so here too even the most loyal followers may limp in a little footsore by the end of the day.

The problem with “The Marches” is that, having crossed England coast to coast along Hadrian’s Wall — more than enough for most travel writers — Stewart then embarks on a series of further walks for some 1,000 miles. The accompanying map looks like a spider’s web. Much of what he later encounters is fascinating but not particularly germane. Now that he is a member of Parliament for the area, he may have felt he had to cover most of his constituency. He certainly doesn’t seem to know how to stop.

Still, like Wordsworth, Stewart brings a humane empathy to his encounters with people and landscape. A walk, he believes, is a kind of miracle that can help him learn, like nothing else, about a nation or himself. He is precisely the sort of companion one would want to travel such a route with: informed, engaged and with a great deal of compassion.

the marches
The Borderland Journey Between England and Scotland

By Rory Stewart

Houghton Mifflin Harcourt.
354 pp. $27