When Terry Cudbird retired after a business career, he could have put up his feet or tended the garden of his comfortable home in Marston. But instead, he put on his walking boots.

First he devised a 4,000-mile walk around the perimeter of France, and wrote a book about his experiences, called Walking The Hexagon.

He said: “I studied French history in France as a student and I have always loved long-distance walking. I did a bit of reading and decided to walk around the circumference — I got the idea from the Tour de France, in a way. My elderly parents were rather ill, so I had to keep returning to Britain, but I finished it in the end.”

A former marketing director of WH Smith, he says he is never happier than when poring over maps, calculating distances and making timetables. It took him two years to complete, mostly in one-month stretches.

The book is based on the diary he kept, and accounts of adventurous scrambles in the Pyrenees or hair-raising ascents in the Alps are punctuated with his mobile-phone ringtone, and difficult news from home.

His journey ended in Lourdes, with his mother confined to a wheelchair in a care home and his father — who once jumped down the scree of Great Gable in the heart of the Lake District — also in a home.

He says his escape around France kept him sane and opened up new perspectives.

“The slow rhythm of walking was like a silent meditation, through which I gradually became reconciled to the past.”

So it is no surprise that when Walking The Hexagon was finished, he started planning an even more difficult task — to follow the route of a French infantryman involved in the Allies’ retreat from Belgium after the Germans invaded in 1914. The result is his latest book, Walking The Retreat.

The idea was inspired by a classic war history, The Guns of August by US author Barbara Tuchman. She describes how the two-week retreat ended with the Allies uniting with troops transported from Paris by taxi to attack the Germans in the First Battle of the Marne.

Mr Cudbird said: “The Guns of August is a very dramatic story of the opening weeks of the war, which really gets you hooked. It showed how people could slip into war without realising, and it is said that John F Kennedy read it in the sixties, before the Cuban Missile Crisis, and tried to avoid making the same mistakes.”

He wanted to find out what it was like for the retreating armies, and why, in the end, the German troops did not march into Paris, as they did in the Second World War. Instead, both sides settled into the trenches of the Western Front, which consumed a generation of young men over the next four years.

“I started reading letters and diaries, and became amazed by their resilience. The story of this retreat had never really been told properly. There is a lot about battles, but I am not fascinated by the calibre of guns — I am more interested in the human story.

“I thought it would be interesting to tell the story and walk the route of the retreat.”

While many diaries and letters gave vivid vignettes of Army life, he focused on the diary of one French soldier, André Benoit, a Second Lieutenant in the 67th Infantry, who wrote his account in a German prisoner of war camp, when the events were fresh in his mind.

Mr Cudbird had found a soulmate.

“It is a day-by-day account that says where he was, even down to the exact church, and where he slept. It turned out the man was a mathematician. He did not have a very dominating personality, unlike some of the other diarists, but he had the detail. I decided to make his diary the focus of my walk.”

Months of planning ensued, and he suggested the idea to his publisher, James Ferguson of Oxford-based Signal Books.

“I was a bit shocked when Jim gave me the deadline. He said, ‘You have got to write it in time for the 100th anniversary’.”

He set off last August, equipped with a huge set of 1:250000 maps.

“There is no long-distance footpath that covers this, so it was a question of making it up. There are some local footpaths and I used farm tracks. The landscape of northern France has not changed that dramatically. I met one motorway and one TGV railway line, but otherwise it looked physically like it did 100 years ago.”

His rucksack weighed 13 kilograms, half the weight of a 1914 infantryman’s pack. He also drew the line at sleeping in ditches, as the English Tommies and their French allies did.

“I suppose if I had been authentic I would have slept in a ditch. But André Benoit was 25. They slept in barns or on the ground. I suppose it was August and it was unusually hot, but they would have hardly got any sleep.”

Mr Cudbird travelled in comfort on the Eurostar, then a local train to Charleroi in Belgium. But the start of his trip doesn’t sound like most people’s idea of an idyllic walking holiday.

His path starts under the concrete pillars of intersecting freeways, a rusting ironworks furnace protruding on the skyline. Things look up when he reaches the leafy suburbs and a riverside path.

“I knew I would not get a meal that evening so like the soldiers of 1914, I ate well when I could; in this case tartare of salmon and pavé of beef, washed down with Chimay blue beer.”

Later drinks included rosé and Medoc, and Mr Cudbird discovered that the Germans had raided the wine cellars of French farmhouses they requisitioned, perhaps hampering their fighting efficiency.

At his first bed and breakfast in a Belgian farmhouse, his host showed his collection of First World War books and photographs — the first of many people who shared their families’ memories of the war.

Mr Cudbird had copied key passages from André’s diary and read them each night.

“They had to shelter in ditches when they heard German guns, drag guns up steep slopes, cross bridges and then blow them up so the Germans could not follow. He says the countryside was so monotonous, but they just had to plod on. That was the same for me. It was jolly hot and I was pleased when I saw a solitary tree, just like André.”

Many farmers had been called up, and others left when they realised the Germans were advancing, refugees mixing with the retreating troops. So the August 1914 countryside was deserted, as it often was for Mr Cudbird, since rural unemployment has left many villages with shrinking populations.

He relieved the monotony of his walk by tracking down some of the places André had written about. “In one town they had to find a cart and found one in a factory. The sugar-beet factory is still there. There were once 100 beet factories in northern France. Now there are three.”

As André dodges German shells and encourages his wilting soldiers, he is told that six men have died. The men have marched 30 kilometres in pouring rain and say they can’t go on. He finds a bar, downs several anisettes and pours Cointreau into his flask.

After the Battle of Guise, he had eight men left out of 32. Meanwhile, our 21st-century hero is tucking into rabbit terrine and steak, with local wine, discussing the decline of rural France with the proprietor.

When he reaches the Marne, near André’s home town, he discovers that his bed and breakfast hosts have read his previous. Hearing his new mission, they give him contact details of local historians, and the trail eventually leads to a genealogist who put him in touch with André’s descendants.

After a convivial dinner, “I thought about André sleeping somewhere on the road below and wondered whether he thought he had come home. He must have had mixed emotions.”

André slept in an orchard to be greeted next morning with the order to face north. “We are going to turn round. It is the end of the retreat.”

Walking the Retreat is published by Signal at £12.99.