I am not a hiker. Until a week ago, I didn't even own a pair of walking boots, and the ones I've got still have that delicious new-leather smell that makes me want to press them to my face rather than stamp them down in a puddle. My decision to walk the Thames Path was born more from boredom than any noble desire to get fit. A few months ago, while working in Richmond doing a job I absolutely hated, I'd spend every lunch hour going down to the river and walking as far as I could and dream of having the freedom to carry on.

However, an hour's stroll is quite different from a week-long hike, so I set about making things easier for myself. Rather than walk the 184-mile path in its entirety, I opt for a 103-mile section, from Goring in Oxfordshire to the Thames Barrier in east London. There's also no way I'm going to end each 20-mile day by putting up a tent and eating undercooked sausages straight from a pan. I want a daily reward. So, before I set off, I book rooms in the best boutique hotels I can find along the route. My humming feet are to be bathed nightly in hot water infused with de luxe toiletries, and food is to be prepared for me under stars of the Michelin variety.

I arrive in Goring from Paddington and walk to the Miller of Mansfield. This 18th-century inn has a cosy front bar where wooden tables and stools sit alongside leather wing chairs, and real ale is dished out to guests and locals alike. The restaurant is a more sedate space of white-and-cream decor.

I'll need fuel for the next day, so I opt for the six-course tasting menu; a fish broth, terrine of foie gras, scallops on ratatouille, monkfish with mash and snails, three cuts of pork and a selection of desserts. Full of food and trepidation, I retire to my room, which is a stylish combination of Art Deco furniture and silver flock wallpaper.

My first day of walking starts pretty well. I am enjoying the feeling of being somehow linked to the river. I love how its current points the direction I must go. This sparkling, fast-flowing conduit, lined with bristling reeds and criss-crossed by ducks and swans, is the same river that appears, later in life, greyer and fatter, to wheeze its way through central London. I feel lucky to see it in its first flush of youth.

I walk through woods and meadows into chocolate-box villages where honey-coloured country churches stand over velvet-textured village greens. The river gurgles over its stony bed like children blowing into milk shakes. It's as English as vicars on bicycles. Less than an hour later, this all changes. Signposts direct me away from the river and on to the main road towards Reading. This is the other England, roads lined by boarded-up off licences, greasy spoons and tattoo parlours, where flags of St George - red crosses bleached orange by the sun - hang raggedly from windows. It's as though I've taken the wrong turn in a gallery and walked straight from a Millais exhibition into a Martin Parr one.

As I wander round an industrial estate, I kick myself for not bringing a map. I had imagined a clearly-marked waterside stroll. It's a big river, for God's sake. How could I have a problem following that? But I haven't seen a Thames Path sign since leaving Purley and the only trail on offer is one of blood, presumably from a fight the previous night, splattered along the pavement. My girlfriend, Sarah, at her desk in London, gets the first of many phone calls.

'I'm lost,' I say. 'Can you go online and tell me how to get through Reading?'

'This morning you said you felt all connected to the river,' she says smugly. 'You said it had a primal pull on you.'

'Shut up,' I say crossly.

I make a quick loo stop at the John Lewis in Reading, and soon afterwards the Thames Path returns to the willow-draped, reed-fringed beauty of the morning. When, after nine weary hours, I catch my first sight of Henley-on-Thames, my resting place for the night, my spirits soar. A mere 20 minutes after checking into the Hotel du Vin, I'm lying back in a roll-top bath, hot water stinging my feet and soothing my aches.

Next day, I am in teeth-gritting pain. Blisters and bruises appeased by moisturiser and soft sheets are squeezed back into unforgiving boots. Indeed, when I leave a Marlow pub after lunch, my joints have seized up so much I'm hobbling like an old man. Well, that's not strictly true: an old man, leaning on his stick, passes me with ease as he makes his way down to the river.

So it is with absolute delight that I enter the manicured grounds of Cliveden at twilight. As I approach the colonnaded main entrance, flanked by parked Porsches and Bentleys, I am approached by a uniformed doorman. 'Do you have a reservation here, Sir?' he asks, eyeing my mud-flecked, sweat-soaked body, and obviously trying to work out whether to usher me in or unleash the hounds.

Cliveden is all about grandeur and its vast entrance hall buzzes with staff in black tie and tails scurrying about beneath high oak-panelled ceilings to bring pots of tea to families with Michael Heseltine hair. Four American children in identical pyjamas run past oils and suits of armour to pelt up a sweeping mahogany staircase. It's as though I've walked into an E Nesbit novel.

Sarah joins me for the weekend section of the walk, and we sit on a tapestry sofa so I can show her my blisters over a glass of pink champagne. Having been politely declined entry to the hotel's Michelin-starred restaurant due to the fact I'd failed to pack a jacket and tie in my rucksack, we drink a bit too much Pouilly-Fumé in the Terrace Dining Room and wake up groggy. Not ideal. We've got more than 20 miles to cover. Sarah asks the reception staff for a map of the area, but it's a bit lacking in detail.

'I don't think this can be the right way,' I say. 'I haven't seen any Thames Path signs for ages.'

'Of course it is,' says Sarah. 'Everyone we've asked has said this is the way to Windsor.'

'Yes, but the river's flowing in the wrong direction.'

It turns out that we've somehow got on to a man-made stretch of river, complete with characterless grass banks and purpose-laid gravel paths, which runs parallel to the Thames. I'm loving having Sarah walking alongside me, though. I never realised how much walkers need companionship to spur them on.

We decide to improvise and, after asking directions in a pub in Dorney, we off-road across a common and rejoin the river at Eton Wick. The outline of Windsor Castle dominates the horizon, and the Thames starts to feel less like the rural wild child it did 40 miles back and more like the middle-aged urbanite familiar to me. We see our first passenger boat, and jumbo jets descending into Heathrow. You can almost smell the capital downstream.

The next stage, from Richmond into London, is the bit I've been most looking forward to. This is the river I know, and the water, which in Oxfordshire was undisturbed bar the occasional duck dabble, now teems with traffic. Rowing boats, canoes, police launches, pleasure cruisers and cargo-piled barges cut their way through granite-grey water.

The Thames here bisects the capital like a seismic-detection graph, and its curves, imprinted on the national consciousness thanks to their central role in the tube map and the EastEnders titles, wind us through Mortlake, Chiswick and Barnes, towards Pimlico and Westminster.

We leave the Thames at Westminster and take a bus to the Haymarket Hotel. The proximity of public transport throughout the day, however, has caused ructions. Sarah, exhausted and sodden due to a sudden downpour around Kew, struggles to understand my need to walk the path in its entirety when we could cut out several miles by simply taking the underground. My Shackleton-esque zeal to complete the task started to grate somewhere around Fulham, and we've barely spoken since then.

I notice a thaw, though, as we enter the hotel's calm, welcoming lobby, and leave the cold and driving rain outside. The junior suite we are led to - a tranquil, white space filled with soft, floral-print sofas and chairs, and an enormous canopy bed - lifts our spirits further and we order a bottle of champagne on room service. Within half an hour we are sitting in a piping-hot bath, glasses in hand, watching The Simpsons on a plasma-screen TV built into the wall. I feel more decadent than a French duke circa 1788.

On the final day, I travel into work with Sarah and leave my rucksack next to her desk. Today is the final push, and I'm looking forward to walking the last section of river without feeling I'm dragging an anvil behind me.

Between Westminster and Tower Bridge, I'm walking on autopilot, weaving through crowds around the London Eye and Tate Britain. But after Southwark, it's new to me. The Thames Path alternates between riverbank and residential road, dipping regularly into the council estates of Bermondsey, Rotherhithe and Deptford, as it takes me past the Isle of Dogs and nearer to the cloud-scraping towers at Canary Wharf.

There's no one around, bar the occasional pitbull-dragging teenager, and I'm struck by how different this final stretch of the river is. This is where the Thames goes to work - except there's not that much work for it to do these days. Everywhere between Southwark and Greenwich I see boarded-up wharves and warehouses. Some have been converted into flats, but just as many have been left to rot.

Between Greenwich and the Millennium Dome, the path is narrow, graffiti-flanked and more than a little eerie. In four years' time, this desolate peninsula will no doubt be some sparkling Olympic park, filled with an international crowd relaxing between triple jumps, but today it looks like a landscape in the grip of a nuclear winter. Empty stretches of gouged soil and crumbling concrete offer no shelter from the freezing wind and the stench of sulphur belches from a factory that would be more at home in some backwater of the former Soviet Union.

I am pleased, then, to round the Dome and get on to the last few yards of the path I've followed for more than 100 miles. It's starting to get dark, and the first sighting I get of the Thames Barrier is of illuminated red crosses on its base, piercing the dusk like cats' eyes. The seven gates, part futuristic spaceship, part Norman ecclesiastical arch, stand like silent silver sentries. There is no one else around and, as I arrive alongside, I feel every bit as in the middle of nowhere as I did when I set off from Goring five days ago.

I rest my arms on the railings and watch seagulls circle and caw around the silvery tips. I'm tired, happy and suddenly looking forward to a celebratory drink. I dig out my phone and call Sarah. 'Darling,' I say tiredly. 'Can you go on the internet and tell me where the nearest bus stop is?'

· Rufus Purdy is editor of the 'Mr & Mrs Smith' boutique hotel guides (mrandmrssmith.com).

Essentials

The full, 184-mile Thames Path, opened in 1996, goes from the river's source near Kemble, Gloucestershire, to the Thames Barrier in east London. It doesn't always run alongside the river, so make sure you take an Ordnance Survey map with the path marked to avoid confusion. Though the path is well signposted, you can't rely on these completely - some have been ripped off or altered by vandals. For more information, visit nationaltrail.co.uk/ThamesPath.

Rufus stayed at the following hotels: The Miller of Mansfield, Goring (doubles from £110); Hotel du Vin Henley, Henley-on-Thames (from £145); Cliveden, Taplow (from £240); The Bingham, Richmond (from £180); Haymarket Hotel, central London (from £250). All hotels can be booked through Mr & Mrs Smith (0845 034 0700; mrandmrssmith.com).