I expected many things, but not the thing about the Bank of England and the M11. Insights into the appeal of London, anecdotes and trivia about its past and present, eavesdropped conversations and glimpses of the people that make our capital city the best human zoo in the world... yes, all of these things I'd expected when I set out to walk the entire Tube system overground. And all of these things I got. But I also got the explanation as to why the Old Lady of Threadneedle Street won't let you join the M11 northbound at Junction 5. Just goes to show – you never can tell where a journey will lead.
It all started on the Bakerloo Line. As the trusty old beast carried me northwards one day, I mused on the walks that had carried me around London, both when I lived there and now that I didn't. Shanks's pony is the only way to get to know a city (one of the reasons Los Angeles can never really be one – you have to drive everywhere). And after you've got to know it, you want to know it better, so your walks get more creative. Mine had included a walk between all the places in London I had ever lived, the route of the London Marathon, the Congestion Charge boundary, Lord's to the Oval... But the real biggie still eluded me. Still there was a desire for the definitive London walk, the one that would truly "capture" the city. How to do it without walking every street, though, which would clearly be impossible?
Staring at the Tube map in the carriage, I imagined walking the journey I was on at the moment (Waterloo to Baker Street). Then I imagined carrying on all the way up to Harrow and Wealdstone, the end of the line. Then I thought of all the other lines. And before I knew it, the plan had formed: walk the whole Tube system. (Nearest streets, obviously, rather than the tracks themselves, which would have been both pointless and suicidal.) This was it, the walk I'd dreamt of. This was all of London without being all of London.
Preparations were pretty minimal: comfy pair of trainers, A-Z, notebook. Even as I gathered together that simple list, though, it struck me that the project would take me to places I hadn't envisaged. Both literally – the ends of several lines were outside the A-Z, requiring more maps – and intellectually; my nerdish excitement at the thought of those maps opened up the question of why? Why did these simple documents that told you how to get from one to place to another hold such a fascination?
The first part of the answer came on a trip to Stanfords in Covent Garden, the travel bookshop that's not so much a shop as the Mecca of maps. It's where Sherlock Holmes buys his map of Dartmoor in The Hound of the Baskervilles, and where a young Kenneth Williams worked as a draughtsman. It now employs Piotr Czapik, who not only helped me find the extra maps I needed but also helped explain how maps are more than just tools for getting around. "They're works of art, not of science," he said. They take liberties with the truth – the A-Z, for example, makes major streets wider than they really are, so you can pick them out more easily. It also includes "ghost streets" – ones that are deliberately misnamed so the publishers can tell whether anyone is stealing their copyright.
Another map moment came later in the project, as artist and ex-pop star Bill Drummond (he of the KLF, who burnt £1m) joined me for a section of the Metropolitan Line walk. He loves maps because "they let you look down from the vantage point of God". The first map he can remember is one hanging in his childhood home in Scotland, showing the local area, which he assumed God must have drawn because He was the only one who could see the world like that. As our conversation progressed I developed a nagging worry that my Tube odyssey might be evidence of a subconscious desire to play God. I was really loving all the cross-referencing between the Underground map and the A-Z, all the marking off of my progress in black Magic Marker. Bill understood. "It's a very male thing to say: 'I can survey all this, I'm higher than the Tower of Babel.' Man's always wanted to build towers, but even the tallest one isn't as good as the 'up here' feeling you get from a map." Oh well. If you've got a God complex it's probably as well that you know about it.
Right from the start, as I set off on the first walk (the Victoria Line, from Brixton to Walthamstow), the experience proved that Dr Johnson was right when he called London "the school for studying life". It's the variety of the capital's inhabitants that makes watching them so much fun. There was the female pensioner astride a 1970s Suzuki motorbike, waiting at traffic lights outside Buckingham Palace, a cigarette hanging vertically from her bottom lip; as she pulled away I saw that the back of her denim jacket read: "The Clash". There were the two men smoking outside a bar near Old Street, both stony-faced, both dressed in lederhosen. There was the businessman in Watford on his mobile: "We've been proactive – did you explain that to Gail?" Sometimes you could tell what the story was straight away: the man getting on to the Heathrow Express carrying nothing but a single red rose, for instance. Sometimes things came into focus more gradually. A couple on the Marylebone Road embraced so tightly that they seemed blissfully happy; only as I got closer did I notice her shoulders were shaking, and the nearest turning was Harley Street. Some mysteries were best left unpondered, such as the man saying into his phone: "It'll be an hour-and-a-half before I'm in Romford, Matilda – if you're going to have a bath, have a bath now."
Another source of entertainment was the city itself – its buildings, signage, graffiti. A sheet of paper attached to a lamppost near Acton Town read: "Have you seen this dog? Answers to Zelda. But it's deaf." A builder's ad in k a St John's Wood phone box announced: "We our the perfectionist." There was the van parked near Marble Arch whose side read: "Stephen Fry Plumbing and Heating Ltd", and the garage in Croxley that had been spraypainted with the message: "Gordon Brown? More like fucking clown". You knew you were in an affluent part of Maida Vale when a front door carried a specially made brass plaque engraved with the words "NO JUNK MAIL – PROSZE O NIE WRZUCANIE ZBEDNE POCZTY". (Yes, the last six words really are Polish for the first three.) Meanwhile in Essex, on the Central Line, the stone gateposts of a Footballers' Wives-style residence were carved with the warning 'NO TRESSPASSERS'. You do hope they weren't paying by the letter.
My modus operandi was a line roughly every other week, meaning the total of 11 (none of this Docklands Light Railway or Overground nonsense) took me from July to December. So I got to see London in summer and winter, during the week and at the weekend, even – on the Jubilee Line – overnight (started at Stanmore at 11pm, reached Stratford at 3pm). Four of the walks were two-dayers, and one (the Metropolitan) took three days – three days that happened to fall during all that snow we had just before Christmas. Heathrow closed, but I ploughed on.
On several of the walks I interviewed people who could help unravel the secret of London's charm, or shed light on different aspects of its character. There was Tim Bentinck, who as well as playing David in The Archers was for several years the "Mind the gap" voice of the Piccadilly Line (he dreamt of royalties, only to learn that his agent had agreed a £200 flat fee). Bentinck commented that London's status as a collection of villages which gradually got joined together can still be felt in his profession: all the voiceover studios are gathered in Soho, so community gossip flows there just as freely as in Ambridge. This "village" aspect, the idea that London isn't really one place at all, was also touched on by John Pearson, official biographer of the Kray Twins, who met me in Whitechapel on the District Line walk. "The East End always stood apart," he said. "There was a sense that it was them against outsiders, even outsiders from other parts of London. You can make too much of it – I think the Krays did make too much of it – but it's certainly true that they were local heroes." The capital's parochialism, even greater in the 1960s, helped when the twins had to lie low. "There was a hotel in Finsbury Park they used to go and hide in. The notion that that's so very far from the East End seems incredible, but that's how it was."
Rachel Martin Pe'er, a student of the Knowledge about to get her cabbie's badge, told me about her 19,000 miles on a motorbike memorising London's streets. Peter Rees, planning chief for the Square Mile, told me why London rather than Paris is Europe's financial centre. "The English pub differs from a French café by people standing up to drink. This means you can be closer to people you don't know. The gossip you get in a French café is the gossip of your groups. The gossip you get in a London pub is the gossip of other people's groups. Now that's very valuable, because you can take that back to the office and put two and two together and make some money."
There's something comforting about the Tube map. Its success as a design icon is partly because of its simplicity (Harry Beck's famous insight that it didn't have to be geographically accurate), but also partly, I think, because it's childlike, a set of lines drawn in different colours that call to the crayon-wielding six-year-old in all of us. The map was particularly inspirational for this project – knowing the lines I was following were (on paper if not in reality) largely straight ones, their few curves perfectly geometrical, helped give a real sense of purpose to the walks, as though I was laying waste to London with Zorro-like swishes. Even in areas I knew well there were discoveries: Little Ben, for instance, a 30ft replica of Big Ben outside Victoria Station (how could I have missed it all those years?).
But the real joy came from visiting areas I'd never seen before. I learnt that Wembley boasts several all-Sikh building gangs. That Barons Court has the most beautiful Tube station on the network. And that Arnos Grove is a very nice area. I'd always imagined it to be grotty. Something to do with the name sounding a bit like "Arnold", I suppose. But also a consequence of the Tube map. Those lines, with their evenly spaced dashes denoting the stations, give the whole thing the feel of a statistical chart. You can follow the Piccadilly Line from super-posh Mayfair to down-at-heel Manor House. Arnos Grove is beyond that, so surely it's even more down at heel? Not in the real world it ain't.
The project opened my mind as well as my eyes, for one very simple reason: exercise releases endorphins. The author Henry David Thoreau put it more elegantly when he said, "The moment my legs begin to move, my thoughts begin to flow," but essentially it's chemistry that's at work here. As soon as those endorphins hit the grey matter, spirits lift and ideas appear. British reserve gets broken down, too. On the District Line, leaving a Chinese restaurant in Upton Park after a meal break, I thanked the owner by saying "Sheh sheh", the only bit of Mandarin I know apart from "Nee-how" ("hello"). I always want to do this but never have the courage. I was glad I did it this time; the owner's delighted smile showed how pleased she was. Only as I set off towards East Ham did I realise that it was the endorphins that had made me do it. They're better than drugs. Or at least up to the 30-mile mark they are. Any further (I had several days which fell into that category) and tiredness reclaims you. My longest day was 39.5 miles on the Piccadilly Line. I felt very low by the end of that.
So what did I learn? What did the 403 miles, the 900,000-odd (very odd) footsteps teach me? Several big lessons about our capital city, such as the fact that its character is changeable. Other big cities have character too, but their character is always the same. Manchester is smirkingly cynical. Birmingham is... well, there's a reason for the old joke about a true Brummie being born within the sound of someone moaning (I know – I grew up near there). But London has mood swings. In fact, it bends its mood to suit whatever your own mood is at the time – it's a mirror rather than a place. London is infinitely malleable as it's made up of so many different people from all over the world, most of them there because they didn't feel at home at home.
As well as the big lessons, though, there were 1,000 small ones, facts and stories and snippets of history from 2,000 years of London. I learnt that the city's cabbies do their first fare for free (for luck), that Nelson is buried directly underneath the centre of St Paul's dome, that the Savoy once laid a rubber roadway between itself and the Strand so guests wouldn't be disturbed by horses' hooves. I learnt that there's no pod 13 on the London Eye (superstition again), that the O2's roof weighs less than the air inside it, that supposedly all-American Marlboro Man took his name from Soho's Great Marlborough Street (one-time site of the Philip Morris factory).
And, oh yes – I learnt, while researching Debden on the Central Line, that the Bank of England's notes are printed out there. As the factory is very near Junction 5 of the M11, the authorities really wouldn't want you heading out into the country if you'd recently paid it an "unannounced" visit. So you can only join the motorway southbound, forcing you back into the capital. In that part of the world at least, all roads really do lead to London.
'Walk the Lines: The London Underground, Overground', by Mark Mason, is published by Random House, priced £12.99. (Follow him on Twitter @WalkTheLinesLDN)