This is the maddest thing you can do to yourself ever. It turns your feet into bleeding stumps, your muscles into concrete and your brain into oatmeal. In the last five minutes, you've pictured yourself in a taxi, just to torture yourself; in your mind's eye, you're having a hot shower and someone is waiting to hand you a fluffy towel and a pair of cashmere socks.

You have been on the hills for 11 hours, clutching your route description, 19 pages of dampening A4, heading for places with League of Gentlemen names like Fang and Drool; whether you get to find them is another matter, because the instructions are written in a language analogous to English. Here's an example: Go thru to X field by fence on Lt to field corner and enter overgrown sunken Trk (B070) to keep AH for 600m thru several Gts (Trk becomes well used after 1st rusty Gt).

You were able understand this at 10am, when you were fresh. But as time wears on and you've been climbing all day and you're suffering from the incremental fuckwittedness caused by the drip-drip-drip of fatigue and cold it's all too easy to go wrong. And then you have to double back, trying to remember which way you came from, worryingly aware that time's getting short. So when you're back on track you force yourself to run with increasingly stiff and painful strides to make sure you reach the next checkpoint before it closes.

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Run. What a nostalgic memory that evokes, like "comfortable" and "really enjoying this". You ran the first 11 miles and after that, when the climbs got steeper and you had to start walking, you were still walking bloody quickly. You're still walking now, but with 40 miles under your feet you're down to a plod. And there's another 60 to go.

You're dawdling along self-pityingly when there's a flash of colour, and a woman of about 25 in a bright red fleece goes belting by, followed by two lads in hiking gear. None of them see you because to them you're completely static, in a boring way like a bin or an old pile of builders' rubble.

Old. That's how you feel. With grating joints and bent shoulders, and a sharp pain in one instep, and big blue veins standing out on your hands.

Oh, get a grip. It's not as though you're past it. For a long-distance walker, you've yet to reach your prime. Boyd Millen, the first man ever to do a double Bob Graham Round back to back, walked 100 miles over the Pennines one January to mark his 60th birthday. In snow. Wearing trainers without any socks. Henry Bridge of Soham – still at it past 80. Some of these people don't take up the sport till they've qualified for the winter fuel allowance.

You plod on, thinking of where you'd be if you weren't out here.

In a line of stationary traffic waiting to go through the Blackwall Tunnel.

Or staring at a computer screen, trying to renew your Norton Internet Security software without signing up by mistake to a different package at nearly three times the price.

Or riding on a Network Southeast train full of drunks eating KFC.

It's no contest, is it?

***

The Hundred is the flagship event of the Long Distance Walkers Association, whose many members meet up most weekends for the simple leisure activity of walking a mere 20, 40, even 60 miles. This is for people who want to go even further – 100 miles non-stop across country, to be completed within 48 hours. It takes place annually on the late Spring Bank Holiday, the venue changing every year from one area of spectacular beauty to the next. It's not a race and there are no prizes, though if you finish you get a badge and certificate. But by Checkpoint 6, only 43 miles into the route, more than 50 of the 500 starters have already dropped out. With 9,000ft of ascent on the first day alone, it's the toughest Hundred anyone can remember.

But then, they say that every year. Anyway, you're not giving up now. You can't. For months, the Hundred and you have been trapped in a love-hate relationship. You've gone on training walks together. 30s, 60s, 50s; you've fought your way through them all. It's beaten you up and reduced you to tears, yet you still keep coming back. A couple of days is all it takes, and you'll forget the bad bits, the sore feet and bramble gouges and liquid cow shit flooding your trainers, and think only of the good things: banter, mates, the feeling of shared suffering and endeavour, the way grass turns pale green by the light of the moon.

Tuna bap. Fistful of raisins from a Tupperware container. Then it's head torch on, and once more into the dark. And now here's another bunch of people. Jim, who's wearing a Newcastle shirt, is on his third attempt to complete. You bond over a discussion about what it is to support delusion-inducing football clubs. Benita, possibly released into the community but somehow fascinating, is kitted out in dangly earrings, Doc Martens and what appear to be baby-doll pyjamas. David, a first timer, is a genial puffed-out ultra runner in Madras-check beach shorts worn with a Berghaus fleece, and Stanley is a magnificent bearded homme d'un certain age in pleated trousers, army boots and schoolteacher jacket. You're in puckered tracksters, smelly thermals and red hat. You're all the people you'd move away from if you sat next to yourself on the Tube.

***

Checkpoint 8. Another nine miles done. In two and three-quarter hours! God, you're rather good at this, actually. You'll be storming into the finish by teatime tomorrow. You might even run the final 100 yards, just to show your complete mastery of the sport. Inside Drool Village Hall, it's warm and welcoming. Jim stuffs himself with cheese straws and peanut-butter sandwiches while telling you this is his third attempt at a Hundred. Both other times he had to drop out at 70 miles.

He's mad. If you've dropped out twice already, you've just got to accept you can't do it. Why punish yourself again?

Oh well. Let's do it. Out into the gripping night. Everyone switches on their head torches again and streams away like a line of miners, heading up into the forest.

Another mile of climbing. Your heart is thudding like the bass in "Walk The Line". Way ahead, a string of pearls is rising slowly into the sky, or so it appears at first gaze. After that you begin faintly to make out a grey-green hump against the backdrop. The tiny pearls are the lights of other competitors' head torches, and the hump is ...

Another climb. An 1,800ft climb. Blithering, blithering God.

Never mind. Here are some more new friends. Andy is on his seventh Hundred. You have incredible hallucinations on the second day, he says; in the 2004 Exmoor he saw a Shakespeare play enacted by a roadside on a brightly-lit stage, with people in doublets and codpieces and plumy hats. When he looked again there was no one there. Nothing but a garage with the doors wide open. Margarita, his partner, who's only on her third Hundred, chips in with the time on the 2003 White Rose when the stones on the path in front of her started moving. She kept having to walk round them. She didn't want to tread on them in case she hurt them.

This is a long, long haul. It just ... keeps on ... going up. And up. Your quad muscles are hurting. The only way to keep climbing is to pile-drive your thighs into the path with the palms of your hands for leverage.

Right. Just take it one mile at a time. Twenty more hours, increasing agony, no sleep, but then it'll be over. You'll have done it! You'll feel great!

No you won't. You'll feel crap. Actually, you might quit at the next checkpoint. You just don't want to do this any more.

***

Checkpoint 9. It's a tent, appearing suddenly out of the pitchy darkness. A frozen hand, its owner partially clad in a sleeping bag, appears out of the vent to clip tallies.

Bugger him, I can't give up here. Not when he's sacrificed his Saturday night for us, huddling under canvas on a cold mountain in the dead of night, just to keep us all going.

Anyway, I don't feel that bad. I can't quit when I'm not even shuffling.

Fumblingly you stuff your tally back into your rucksack and follow the others into the dark. You and Jim have agreed to finish this Hundred together. Celebrate by getting together afterwards next time Newcastle play Spurs. In fact you're practically on the brink of civil partnership when there's a distant zig-zag of lightning, a drum roll of thunder and rain in torrents.

Everyone stops to clamber into waterproofs except for Benita, who puts on a raincoat and shower cap. It takes so long to get your stuff out of the rucksack you're drenched already. As soon as you do up the last zip, the rain stops.

Drip, drip, drip. Moan, moan, moan. You're already lagging behind the others when a bramble as taut as a tripwire catches your foot.

You go flying.

Sit there for a bit, regrouping.

You must be conscious because you can feel rain running down the back of your neck. And because you can feel your shoulder hurting. And because you can't feel your hand, which is numb. "You all right there?" asks Jim. Everyone is standing round, holding out their hands to pull you to your feet.

"Yeah, fine," you mumble. "Just got to sort myself out. I'll catch you up."

You start hunting in your rucksack for the drinks pouch. Then you stop. Yes. This is it. You've had enough of this for one lifetime.

You start throwing sticky wet stuff out. Outdoor Leisure maps the size of loaves. Loo paper for en-route exigencies. Strips of ProPlus. Quadruple-strength painkillers. Energy bars made of budgie seed and minced cardboard. Your phone's covered in bits of fluff and dust, it's the world's first flock mobile.

You won't need any of this. You're quitting.

Thing is, it takes ages to clear out a rucksack. For five minutes you're sitting there, chilling like a Sancerre, strangely fuddled, full of vague indecision; whether you could stay here and wait to be helped, or whether people would just scud past you, heads down, and you'd die and months later they'd find a skeleton in trainers and red hat.

You shine a torch on your hand. Your left palm is turning the colour of Paul McCartney's hair.

But dammit, what kind of wimp are you? It's not as if you're going to be walking on your hands.

Minutes later you've got moving again.

It's a harrowing few minutes, having decided it would be a brilliant idea to catch up the others by running after them. This time you stumble to a halt after 500 yards. Pause button jammed. You pretend you're only stopping to check the route description, but it's no good. You have gone beyond non-enjoyment now. Beyond indifference, beyond dislike.

You hate this sport. HATE it. The most pointless activity known to man. No money in it. No glory. Nobody writes about it in the papers. You don't get a goody bag full of freebies like in big-city marathons. It's not like fell running, all romantic dash and glamour. Even curling isn't as silly as this. At least it's an Olympic event.

You stagger along, considering finding your way to a road in secret. You could say you were just going to "pop into the bushes", then you'd hit asphalt. With judicious use of an OS map you could probably get away with that for quite a few miles. It'd be exciting, like a clandestine tryst.

Only the trouble is, you can't find where you are on the map. And you've got a compass but you're useless with it. You might as well try and find your way with a can opener.

So it's back to the route description. Here goes.

You lumber onwards with Jim, Benita, David and Stanley. You huddle in a sheep field while David and Stanley pore over the map. Time passes at the rate of the Northern Ireland peace talks.

"I think we ought to turn left here," says David, flapping the map like a toreador.

"No," barks Stanley. "It's much further on."

"Trust me, I've reccied this bit," says David.

They march off. You, Jim and Benita trail after them, keeping tabs on them at first by the little bobbing beams of their head torches then when the lights disappear, just trundling morosely forward till you hear raised voices.

Stanley and David are standing at the corner of a field. "You're an absolute bastard," Stanley is roaring. "You led me astray completely. You don't know the difference between the left of a hedge and the right of a hedge. Follow hedge on right. You followed hedge on left."

"I won't be spoken to like this," says David in the tone of a man who requires a door to slam, and storms off.

You, Stanley, Jim and Benita head down the only track you haven't tried and find David sitting on a stile.

"You again," says Stanley. "You bloody bugger."

"I've lost my compass," bleats David.

You can't stand them any longer. You tramp along in the darkness, 10 yards in front of them, buffeted by wind, lashed by rain. Plus the blisters on your left foot are so sore you can't put your heel down. You're walking like Larry Grayson. Night is blanketing the valley as you descend into the village of Fang.

•••

Checkpoint 10. Inside the community centre is a smell of damp tuna sandwiches, embrocation and wet clothes, and a flash of brightness at a corner table where a young woman, flame hair limp and darkened with sweat, tends her feet next to a man in a thermal hat. He looks like Van Gogh in his self-portrait, with that bandaged head and those enormous eyes haunted by defeat and despair. Two bodies are stretched out on the floor.

It's 2am. You start to change your socks, but blood has superglued them to your raw, skinned feet. You squint at the route description. It hurts to read now. "This leg should give you a comfortable ramble on pleasant turf-carpeted paths with short stretches along roads."

Roads! My friend asphalt! I can do this!

Leaving the warmth and light seems one of the hardest things you've ever done. And the road stretches are agonising to your sore, skinned feet. Plodding, stumbling, limping along, you can't bring yourself to talk. You just want to shuffle along, you and your pain.

Your wrist is the size of a Ginsters pasty. It hurts to swing your arm as you walk. And your left leg won't bend any more and someone is trying to sever your right leg below the knee.

You must be near the next checkpoint now, surely. How many more miles? "Two," David mutters through parched lips. Two miles. That's, like, on the moon.

***

Checkpoint 11. Through another godforsaken village you've staggered, past windows where people are drawing back curtains and doors with Sunday papers sticking out of letterboxes. Outside the Scragg Temperance Hall, bodies are propped, heads lolling, spreadeagled against walls, oblivious to brambles and weeds. Someone is sobbing, inconsolable at having had to give up. It's like the aftermath of a shoot-out in a Wild-West saloon. Very slowly and carefully, you weave your way through the bodies and flop into an empty chair. You've come 60.9 miles. There's another 39.1 to go. You can't even face the 0.1.

"Sorry, Jim, I'm quitting."

Blood red of eye, Jim nods at you uncomprehendingly and hobbles off into the dark. He's mad. Look at him. He won't get another mile.

You are not mad, you think to yourself as you wait for the body bus to take you back to the finish. You're being sensible, giving up before you do yourself serious damage. There's always next year.

Yeah, you did pretty well. 60.2 miles. Practically two-and-a-quarter marathons. There's not many who could do that. Three cheers for me!

And half an hour later, you're slumped in a slough of disappointment and self-hate.

You're just a complete and utter wimp. Everyone must despise you. Go on – you know they do. What a ninny.

***

Back at HQ, Sunday turns into Monday. You envy the people who've finished, sleeping with their heads on the table. And the people holding their backs, with lopsided shoulders, and the ones who daren't take their trainers off because of what they might see, and the ones smiling vacantly and talking crap.

The sun is high in the sky by the time the shout goes up that someone else is on the final approach. You step gingerly outside to look. It's a dead man walking. It's Jim, tilted to one side, painfully slow, every step a massive effort of will and determination. *the time it's taken him to get from the 60-mile checkpoint to here he's aged from a man in his mid-40s to someone who looks as though he qualifies for a telegram from the Queen.

He has tears in his eyes, is in that faraway state of weary rapture, experiencing a moment as intense as that felt on the brink of any long-dreamt big-time sporting triumph: think Goran Ivaniševic's lonely ecstasy as he served to win Wimbledon at last, at last. It's been so long coming.

He props himself up in the doorway, pulls himself up with a grimace and drops his tally on the desk. Then he collapses in the nearest chair.

Even though each small movement is agony, you squat down and unlace his trainers with your good hand. It's the least you can do. What a hero.

You're terribly envious. You want it. You want to complete a Hundred so badly. That feeling of, I've done it. Dues paid. You didn't give in. You climbed over the wall of physical and mental torment and slithered down the other side.

There's only one thing for it. You'll have to come back next year and try again.

The Hundred, you big, beautiful bastard. As long as it takes. I'll tame you yet.

Julie Welch went on to complete the 2008 Yordale Hundred.

Epic trails: Britain's greatest walks

Cotswold Way

This week-long walk takes you into the heart of the English countryside, going through pretty Cotswold villages and providing panoramic views of the Severn Vale. Ninety-six per cent of the trail is within a designated Area of Outstanding Natural Beauty, and it runs 102 miles between the market town of Chipping Campden in the north and Bath to the south.

Pembrokeshire Coast Path

A challenging coastal walk which offers plenty of rewards: more than 70 beautiful beaches, 40 Iron-Age forts, and archaeological remains from Neolithic times onwards. It wriggles 186 miles along the coast of south-west Wales, from Cardigan in the north to Tenby in the south, along rugged clifftops, around pretty coves and out on to wide, windswept beaches. The 35,000ft of ascent and descent is equivalent to climbing Everest, and will take between 10 and 15 days.

Southern Upland Way

Go coast-to-coast on Scotland's longest National Trail. Running between Portpatrick in the west to Cockburnspath in the east, the 212-mile walk takes in a variety of terrain, from coastal paths to moorland, forest and hills as it winds its way across the country. This trail has some long and tough-going sections, rising to over 700 metres, and takes up to a fortnight to complete.

Pennine Way

Walk the backbone of England on this epic 268-mile trek following the Pennine mountains. One of the most popular long walks in Britain, it stretches from the Peak District up through the limestone-speckled Yorkshire Dales before crossing Hadrian's Wall to finish in Kirk Yetholm, just over the Scottish border. The Pennine Way was the brainchild of rambler and writer Tom Stephenson, and became the first designated National Trail in 1965.

Ayrshire Coastal Path

Scotland's newest long-distance route, this 100-mile trail along Ayrshire's beaches and clifftop tracks was officially launched last summer. Walkers are accompanied by the ever-changing views of the two mountainous islands, Ailsa Craig and the Isle of Arran, across the Firth of Clyde. Allow between five and 10 days to do justice to this wonderful stretch of west-Scotland coastline.

Offa's Dyke Path

Crossing between England and Wales more than 20 times as it snakes 177 miles along the border, this trail follows the dyke built by King Offa in the 8th Century to divide his lands from rival kingdoms. Linking Sedbury Cliffs on the Severn Estuary with Prestatyn on the shores of the Irish Sea, the walk goes through three different Areas of Outstanding Natural Beauty: the Clwyd hills, the Shropshire hills and the Wye Valley, and usually takes about 12 days to complete.

Julie Welch Book offer

Out on your Feet: The Hallucinatory World of Hundred-Mile Walking is published by Aurum (£16.99). To order a copy for the special price of £15.29 (free P&P) call Independent Books Direct on 08430 600 030, or visit www.independentbooksdirect.co.uk