I don’t remember deciding to walk the old Roman road north, but strange things can happen when you’re wrestling with a novel. All I can recall is that it suddenly seemed imperative, as things often do in dreams. What I really needed was to step into the half-born book and let it close over my head for a few days – perhaps it was this kind of inchoate instinct rather than the more rational reason I gave myself, which was that I should walk at least a little way in my character Jack’s shoes in order to get a feel for the journey he makes.

Melissa Harrison. Photograph: Jason Alden/eyevine.

Jack is a one-time protester, itinerant farm-worker, vagrant, poet and seer who has spent 30 years on the road, and of At Hawthorn Time’s four main characters he’s the one with the least conventional life. When the book opens he has just served half of a prison sentence, the latest in a long line of convictions for trespass; he skips bail to walk up Watling Street, now the A5, in search of work. Jack’s journey north out of London was, for me, uncharted territory.

Not that the busy, workaday suburb of Harpenden felt particularly like the unknown as I disembarked from a train with my backpack one April morning. I was well prepared for the next four days, or so I thought: better than Jack would have been, anyway. I had hiking boots on, and a walking jacket, an Ordnance Survey map and a mobile phone; and I’d booked overnight stays at two pubs and a B&B, wild camping being illegal in England. I had water and cash, and my notebook, and the weather was nice. How hard could walking up Watling Street be?

Pretty hard, it turned out. Being an old Roman road, the A5 runs straight as a ruler and the traffic on it is fast; while the sections through residential areas were easy enough, I soon found that the long stretches between towns were far more difficult to walk along than I’d anticipated. I’d known, of course, that it wouldn’t have pavements for much of the way, but I’d thought the verges would be passable – in fact they were overgrown and tussocky and dangerous to traverse given the lorries thundering past only a few feet away. I’d also assumed that, where necessary, I could walk parallel to the road, on farmland; but the fields were mostly paddocks rather than arable, or fenced off with “Private” or “No Trespassing” signs. Used to hiking in more accessible parts of the countryside – places such as Devon, Hampshire and Cumbria – I was dismayed to find I could not strike out on my own.

The solution, I soon realised, was the meandering public footpaths marked in green on my OS map – but using them to get between each of my overnight stops threatened to add many kilometres to each day’s walk, which worried me. I felt frustrated, too, that the trip I’d planned so carefully was turning into something else altogether; that I wasn’t going to be able to discover things about Jack’s journey in the way I’d expected.

But I began to discover other things: a strange white tower with a cryptic sign that read simply “A”; an old drover’s inn, boarded up, its livestock pounds given over to plant hire and goats; a stream forded by a gimcrack bridge built from scrap timber and trench covers; a fenced-off fragment of land by a motorway with a single, well-kept human grave. Halfway through the trip I finally let go of the walk I’d planned, and found myself able to pay attention to the one I was actually on.

And I wouldn’t have missed it for the world. Being out for that long by myself, navigating my own route and succeeding or failing by my own decisions was exhilarating. I found that while I didn’t feel unsafe I did feel very exposed – and that, in itself, was interesting. It made me realise that for much of my day-to-day life my actions are outwardly intelligible: I’m a commuter, using public transport at the agreed times; someone having an evening drink with friends; a tourist, hiking with my husband in a touristy area. But for those four days I was an anomaly: a lone woman with a backpack, on a weekday, in a part of the country where there were no walkers – and eating alone in the evenings, too. Cars slowed down; I attracted curious and concerned stares, even questions. By the fourth day, tired but triumphant, part of me was longing for my everyday, explicable life back so I could feel one of the crowd again.

Watling Street has left its mark on me – as well as on the book. Many of the things I saw made it into the manuscript, but more importantly, Jack’s journey took on a quality it wouldn’t otherwise have had, because it helped me imagine what it might be like to choose to live outside society’s given roles. My walk was a lesson not just in what it costs but what can be gained by not fitting in; and a reminder that in writing, as in life, not everything can – or should – be under our control.

• At Hawthorn Time by Melissa Harrison is published by Bloomsbury.