It wasn't a good start. I'd queued patiently at the Charing Cross ticket office and now, when I'd finally reached the speak-clearly window, the young clerk could hardly bring himself to issue my single to Erith because he was laughing too much. Laughing aloud. Loud enough to be heard by the people behind me. WH Smith's was no better. The elderly lady at the till, who, as usual, was busy wasting everyone's time by feeding newspaper barcodes through her machine instead of reading the price from the front page, hardly registered my presence until I laid down my four bottles of water. Even then she only glanced up for a second. But it was enough. I could see from the way her lips tightened that she too was fighting off incipient hysteria.

As I settled in my empty compartment on the Erith train I began to think that the real cause of all the silly giggling and boorish laughter which had greeted nearly every one of my public encounters that morning had been the map. I might have got away with the big brown walking boots and the black waterproof over-trousers and the pewter welded-seal over-jacket and the 15 litre polyester rucksack, if only I hadn't also been sporting a plastic Trekmates map case. After all, you can see big boots and waterproofs any day on the streets of London. But only the most naïve tourists wander around with wide-open maps. And even those metropolitan innocents don't typically choose maps which measure a good six inches across from east to west and then hang them upside down around their necks.

But that map was vital. How else was I going to find my way from Erith to Old Bexley, the very first stage of my 150-mile walk around the whole of Greater London, the very first link on the walkers' M25, the London Outer Orbital Path, the celebrated Loop?

My book had told me that there were signs along the way – look for the green kestrel in a white circle – but there was also an ominous reference to sections that had not yet been effectively way-marked, and a footnote which recommended a grown-up compass.

Erith provided my first reassuring moments of the day. There was no one at the barrier of the station to mock my arrival and outside most of the locals seemed far too busy countering the wind blowing up from the Thames to take in the slightly bizarre sight of their familiar High Street being addressed by someone geared up for the nursery slopes of the Himalayas.

It was out on Crayford marshes, though, that I fully came into my own. My waterproofs happily battled and won against the rain sweeping in from the river and my Brasher boots chomped through the mud as though they'd been yearning for the chance to show their mettle.

This was what it was all about. This was the way to see and understand the real London. This was also the time to pay a little mental tribute to the skill and tenacity of all the people who'd helped to put this extraordinary path together. This, I reflected piously, as the marshes gave way to a pretty path alongside the tiny Darent River, was a refreshing triumph of public-spiritedness over private property, proof that civic pride could override the encroachments of global capitalism.

But as I realised only a few miles later, the Loop has the unfortunate habit of luring you into such elaborate rural reflections and then, quite suddenly, hurling you back into suburban reality. One moment you're watching kingfishers on the River Cray and the next you're stepping out from a narrow passageway into a full-blown front lawn double garage development.

If big boots and baggy waterproofs and thermal hats and inverted maps looked out of place in the concourse at Charing Cross, here, in Neighbourhood Watch territory they looked downright threatening. I could only have prompted more net curtain twitching if my rucksack had prominently announced that it was transporting swag.

Somehow, though, I survived the suspicious glances and the menacing dogs and made it to Old Bexley High Street where a group of workmen were digging in a deep pit. I had to halt for a moment to let another pedestrian pass and this gave them all a chance to stare up from below at full hiking regalia. I could hardly brush off such concentrated attention. Some explanation was needed. "I'm on the Loop", I said. The big man nearest to me in the pit nodded kindly. "Of course your are, mate", he said. "Of course you are". And went back to his digging.

Look out for Laurie on his fifth section of the Loop (Hamsey Green to Banstead) a week on Thursday