The final straight — a very long straight, followed by curves, yachts, planes and cable-cars. No, I didn’t cheat…

(ICYMI: on Sat 5 May, the writer walked 36km as the first instalment of a 120km circumnavigation of the capital for charity. These blogs are to publicise the fundraising and consider the vernacular. Full list of entries at the end of each one.)

Leaving behind Bazalgette’s cathedral of sewage and the criminally dumped Euston Arch, we plough on along the Greenway. By now, some six or so hours in, the body that yesterday turned 44 is starting to wonder what the hell’s going on and whether it could please stop soon.

Over the last couple of miles I’ve fallen into stride and conversation with Sian, who’s done a lot more of this sort of thing than me. That’s not much given that this is my first charity ‘endurance’ effort of any kind, but Sian is one of a surprising number of people I speak to who do these events regularly, raising money and awareness but also keeping fit.

Sian was up at 5.30am to catch the train in from Southend; she might not be able to do the second leg of this Mencap Capital Challenge because it clashes with a 100k bike event taking place over two days, although she’s targeted a one-day 100k event “later in the summer”, as if doing it over two days is an easy option. I never ask Sian’s age, but I’d say she’s older than this embarrassingly unfit greenhorn.

Indeed, many of the protagonists are. I came without preconceptions but was pleasantly surprised by the range of ages attempting the 36km walk. Perhaps some have personal reasons for supporting a learning-disability charity, but it seems others enjoy the challenge, the camaraderie, the change of scene.

Personally, it’s good to know that events like the Capital Challenge exist, that I can do some good while walking, talking, exploring, photographing, enquiring, noting and apparently laying the groundwork for 10,000 words of notes.

There aren’t many notes to be made as we trudge the Greenway, its ruthless rectilinearity reducing the horizon to a vanishing point. Again, the repurposing of such places into linear parks is to be applauded, and there are a few people illustrating its utility by walking and cycling along it, but aesthetically this one lacks panache, va-va-voom, frankly any interest.

With literally miles to play with, there is room aplenty for public art, public interaction of any sort, but there’s nothing apart from a few fenced-off community gardening projects. Even the street furniture seems an after-thought: more of the signposts are unfinished than finished, which hardly dispels the feeling that this is a road to nowhere; and although there are streetlamps, presumably as a safety feature, there are no benches. (Perhaps they’re fearful of youths gathering, as if attracted by the thought of a nice sit down.) For the majority of its length it’s just an eight-foot wide tarmac path with 25 feet of gently sloping patchy grass alongside.

I’m starting to use the grass a bit more, as it’s more merciful on the soles than the hot interminable tarmac. Sian announces she’s off for a little slow-paced jog: “It uses a different muscle set”. Off she shuffles; I watch awhile, wrestle with whether or not to try it, acquiesce to experience, set off and resemble a bad actor imitating a drunk with possible broken limbs. I’m suddenly glad the Greenway is relatively empty.

So it’s back to walking pace, accompanied by a frequent re-examining of the map as we inch — no, millimetre — along the dead straight red line. With the Greenway slightly elevated, we have a decent view of the surroundings, but they’re the unprepossessing urban sprawl of Plaistow and West Ham.

Instead I focus on reaching the next crossing: at regular intervals, the path is interrupted by intersecting roads. Approaching one of these, it strikes me as odd that the roads rise on each side to meet and crest the Greenway: why would they do that, if this was an old railway or (less likely) canal?

It’s only in later research that the answer arrives with forehead-slapping obviousness. The cargo carried along this arrow-straight line between Abbey Mills Pumping Station and Beckton Sewage Works was human waste. We’ve been walking along an enormous sewage pipe.

Right from where we joined it immediately to the south of the London Stadium, the Greenway follows the Northern Outfall Sewer, Bazalgette’s way of getting rid of the effluent collected by that sewage system. Like the Amazon gathering tributaries, the NOS feeds off smaller sewers coming downhill from Hampstead via Hackney, Kilburn via King’s Cross, Kensal Green via Clerkenwell, Ravenscourt Park via the Strand and Hammersmith via Chelsea and the Thames Embankment. The whole system is mostly powered by gravity, but the latter two “low-level” sewers need to be raised up 12m to meet the others — hence Abbey Mills pumping station, which lifts the contents up to slide downhill to Beckton.

Then what? Well, for the record, here’s what happened; if you’re eating, you may wish to skip this paragraph. At the Outfall Works (and its south-bank partner Crossness), fleets of vessels — we may as well call them shit-ships — were filled with what the industry calls “sludge” and sailed off down the Thames, past the aptly-named Foulness Point, and simply dumped four million tons per year into the sea. Obviously this didn’t last forever — we’re not animals. The EU banned it in 1998. (It’s now compressed, dried and burnt to generate electricity: the light by which you read on the toilet may be powered by your previous efforts.)

With a 90-degree right turn as abrupt as it is welcome, we finally leave the Greenway. Passing through a houisng estate, we walk past Roman Road primary school — the name is presumably a reference to the Colchester highway; the school was built on the site of an isolation hospital for sufferers of smallpox and other infectious diseases. Turning south off Roman Road itself we’re hit by the low boom of a major road, and indeed running parallel to Roman Road 50 metres south is the A13, as commemorated by Billy Bragg in his Essex-flavoured version of Route 66.

Crossing the thundering A13 feels good: unlike the Big-Nosed Bard of Barking, I have no desire to head east so the path’s renewed southerly direction feels welcome. Helpfully, there’s also a couple of attractive parks to go through: Beckton District Park North and Beckton District Park South may lack a little in nominative pizzazz but they’re pleasantly winding places laced with unusual trees.

Then, after faithfully following it since all the way back at the Wembley Sailing Club, 30-odd kilometres and seven hours ago, we finally abandon the Capital Ring. Instead, there’s a short march up the slope of a wide tree-flanked pathway, a left turn — and an ecstatic glint of water. Kids of a certain generation will remember the buzz of being first to see the sea, knowing that the aquatic end-goal is literally in sight; so it is with us and the alluring azure of the Royal Albert Dock, which we know is right next to the Royal Victoria Dock, at the end of which is the finishing line.

With that goal glinting in the middle distance, we fairly gallop over the bridge across the hidden docks road and under the DLR track. Across the water in the heat-haze there’s a sizeable jet plane, then another, and another: London City Airport, boldly carved out of the unused space between the Royal Albert and King George V Docks, and one of the more difficult runways to land on (because of the proximity of tall buildings, the glidepath is 5.5˚ compared to the standard 3˚).

Turning right and heading west into the searing sun, we catch up with a couple of walkers who have just decided they need a drink: “Water?” “No, lager.” They find a bar overlooking the dock, but we press on for one last hard mile.

As the Albert becomes the Victoria, we start to look for the finish line, across the gleaming water. We skirt the vast bulk of the Excel exhibition centre, outside which a passing shirtless youth enjoying a Stella Artois wishes us well: “Keep going, you’re nearly there.”

We pass the Royal Victoria Bridge, opened in 1998 with a 50ft clearance for boats. These days that doesn’t tedn to mean steamships, for which the Victoria Dock was the world’s first to be specifically built (it was also the first to have integral railway lines direct to the quayside), but yachts. One such is moored just west of the Excel: the Sunborn, a 420ft beauty converted into a hotel.

Or was it? Actually, this Finnish-built yacht never graced the ocean waves — it was pulled by tug across the North Sea because it doesn’t even have an engine. The nearby cranes, repainted and beautified since they were last regularly used in anger in the late 1970s, have the good grace not to look too disgusted: the economy’s new engine is not industry but leisure. The Millennium Mills, bulking broodingly and beautifully across on the dock’s south bank, know this: around them, Silvertown is experiencing a £3.5bn facelift, and it will be their turn soon.

As for us, our journey is nearly over. All that remains is to round the corner of the dock, bypassing the queues for the Emirates cable-car over the Thames, and find our way to the finishing line. We’re cheered across it, Sian opting to jog while I prefer to dance. While cheerful Mencap helpers ply us with recuperative drinks, snacks and a medal, we experience the exquisite joy of taking the weight off our feet, soaking up the sun and watching as, 15 minutes later, our friends from the other end of the dock cross the line with lager in hand.

There are cheers, there are thanks, and there are more than one or two aches and pains. But it has been worth it. Three Saturdays hence, some of us will be back again to start the next quarter of the capital circumnavigation — but not before a very long bath indeed.

The Mencap Capital Challenge is a charity walk circling London in four quarters, each roughly 30km. You can donate or sponsor the writer at justgiving.com/garyparkinson1974. You can also join in: the East quarter (Royal Albert Docks to Crystal Palace) will take place on Sat 26 May, the South quarter (Crystal Palace to Richmond) on Sat 9 June, the West quarter (Richmond to Hendon) on Sat 7 July.

NORTHERN QUARTER, SAT 5 MAY 2018
• Pt1: Wembley to East Finchley • Pt2: …to Finsbury Park
• Pt3: …to Clissold Park • Pt4: …to Springfield Park
• Pt5: …to Hackney Wick • Pt6: …to Bow Back River
• Pt7: …to Channelsea River • Pt8: …to Royal Victoria Dock

EASTERN QUARTER, SAT 26 MAY 2018
• Pt9: Victoria Dock to North Woolwich Pier • Pt10: …to Maryon Park
• Pt11: …to Hornfair Park • Pt12: …to Oxleas Wood
• Pt13: …to Eltham Palace • Pt14: …to Grove Park
• Pt15: …to Beckenham Place Park • Pt16: …to Penge
• Pt17: …to Crystal Palace Park • Pt18: …to Upper Norwood

SOUTHERN QUARTER, SAT 9 JUN 2018
• Pt19: Crystal Palace to Beulah Hill • Pt20: …to Streatham Common
• Pt21: …to Tooting Bec • Pt22: …to Wandsworth Common
• Pt23: …to Wimbledon Park • Pt24: …to Richmond Bridge

WESTERN QUARTER, SAT 7 JUL 2018
• Pt25: Richmond Bridge to Isleworth • Pt26: …to Hanwell Locks
• Pt27: …to Horsenden Hill • Pt28: …to Sudbury Hill
• Pt29: …to Wembley, and that's that