"You must be mad!" friends spluttered. Mad enough to ride from St Petersburg to Moscow, 625 kilometres, much of it off-road, in the Russian outback where flush toilets are unheard of. In seven days. Mostly camping. "You do mean motorbikes, don't you?" was also typical. And no, we didn't.

It wasn't as if the Joneses are keen on energetic activity. I'm an overweight asthmatic and JJ's a 61-year-old 50-a-day smoker who prefers fishing. We live in rural Surrey where cars scream by on motorways, never pausing for the beauty, so getting fit on pushbikes might be fun. It would be for the charity Scope, for people with cerebral palsy. And we would not be alone.

At the Hotel Rossiya in St Petersburg gather seventy volunteers, of all shapes and sizes; JJ is the oldest man. Patricia is a red-haired Scottish farmer and a curling coach. Tanya makes wedding dresses for a hobby. Australian Peta is a theatre sister; Tash is a beringed hippie who joins JJ on fag breaks. Chris is in his final year of medical studies. Mark works for Toyota in Wales. "Slug" is swarthily handsome with a pony tail, eleven earrings, and a smashed leg from a motorbike accident. Rebekah, only 4'10", is from West Bengal, a trained acupunctrist (which turned out useful). Middle-aged mums like Carol, Ann and Sue abound. Six Americans have flown over, four from Pennsylvania and two grizzled old buddies from California who've crossed America by bike; they saw the ad in the Sunday Times. Above everyone towers Kiwi Jon Summers. He plays rugby for Reading and is huge, with massive biceps and pecs to die for. Plus assorted teachers, PE instructors and bike freaks.

Our hosts are the St Petersburg Cycling Club led by Ilya I, a slight, bearded man who works for a Finnish oil company. Ilya II is professor of physics at the university, Sergei was a nuclear physicist in the Ukraine, Arkady the doctor is a neurologist in his spare time. In charge are Sarah from Scope and Di Ellington, the Flying Dart. She's a tiny six-stone former world champion fell runner, who spends days speeding through the pack, problem chasing. "It's easy!" the Russians lie enthusiastically. "Flat between here and Moscow. Don't worry about the heat. Drink enough water and you won't have any problems."

We wuzz conned. The first day was indeed easy - in buses, south to Staraya Russia, necessary to reduce the ride to manageable proportions. There's a pleasant surprise: row upon row of shiny new bikes. JJ and I fiddle with saddles, trying to look professional.

The route meanders south-east along rivers into the Valdai heights. It is hot: 90°F by mid-morning. A bike creates its own cooling wind, but whenever we stop heat blazes up from the road, tarmac melts onto the tyres. The shade under trees is like a fridge. By lunchtime, as the others splash in a nearby pool to cool off, Pat with her fair skin is in trouble. She isn't drinking enough, the doctor says; dehydration happens quickly. "Drink!" is the command we all give each other, and "Get out of the sun!" "Have you peed yet?" is a variant. One veteran says that dehydration shows in dark-coloured urine. "What colour is it?" we yell at each other, as the week wears on.

Next day Pat sloshes her way through seven litres of water, and is fine. It's from roadside springs and is fabulous. The young Americans slurp isotonic solution by the gallon and make themselves sick; Arkady shakes his head. When I kiss JJ's bare shoulder (he adores the sun) he tastes like salt and vinegar crisps.

In two days we cover 150km without mishap and arrive at the Pola river campsite. It's lovely, though the tents are tiny. The sky is light till nearly 11pm but we're too knackered to do more than eat, spray the tent for mozzies and collapse. The sturdier bikers are singing round the campfire; we fall asleep to surreal warbles of American Pie in the balmy night.

The dawn is magic. A gentle sun slants mistily through the pines. Tea is brewing, wood smoke drifts through the camp. Porridge with condensed milk is wonderful. Bread comes with Nutella, jam, cheese and salami. I will eat more cheese this week than in a year; Big Jon says the same for the meat, usually stroganoff, which appears at nearly every meal. We're burning up around 4,000 calories a day so we'd eat anything - in fact, it's not bad, the monotony relieved by tomatoes, cucumber and water melon. Each camp site is ready when we arrive and has three huge holes, one for rubbish, two for the privies. The men have to be reminded what the heap of sand is for; we ladies giggle, ours is nice and tidy.

The hard riding starts. In goes the vaseline. Day four is only 50 km but murderously uphill. Buzzards circle. On water towers storks flap and clack. Babushkas in faded flowery dresses sit beside their tumble-down cottages and gape in amazement. "Dobre din!" (good day) we call, and "Dobre!" they reply. Day five (Ostashkov to Kamenista) covers 90 km, day six to Chuprunovo it's 112 km. We join the great sleepy Volga and camp beside it twice. In Rhzev, a prison town, we keep our heads down and don't dismount. I learn to fear instructions such as "end of tarmac" and "sand pit." It's scorching, especially on rutted treeless tracks in the middle of nowhere.

The back markers are struggling. I ride next to Tanya, who is a hefty lass. At each stroke her thighs rub together and her face is streaked with sweat. "Everything's hurting," she grunts. "But I am not getting off." Rebekah rides doggedly in too high a gear, knees out, insteps on pedals. She "feels safer" that way. She has only practised in Bushey Park and has never cycled on a road before. At stops she performs acupuncture on herself, and shoulder massage for the rest of us. Ann keels over and ends up in the van, visibly shocked. Di's riding with a cracked rib and has dark circles under her eyes. At one point we're two hours behind the leaders and feeling very cross at our abandonment, as the dusty road stretches ahead with not a soul in sight.

Next day Sarah speaks out at breakfast: "Hey, guys, this is not a race. It takes real courage at the back. Come and help." The front runners simply hadn't realised; they promptly organise a rota. Chris the young medic is everywhere, helping, cajoling, encouraging; he is a STAR. Mark proves capable of getting legless on 10% Russian beer at night then pushing us along next day; he's reassuringly barmy. Big Jon zooms up, puts a hand on Tanya's back, whizzes her up hills, returns for Ann and Carol, wisecracking as he goes. He's working like a Trojan. Every morning he warms us up with muscle stretching. We beg him to teach us a haka, but he is half Maori and shakes his head.

As we peddle companionably, I ask, "Why are you doing this?" For many it's a life-change. The husband walked out, the boyfriend cancelled the wedding, a loved one died. The kids are about to leave home and it's a chance to prove something, vague but obvious. For several young mothers, I fear for their children on their return: "Don't you mess with me, kiddo," they'll say, "I rode six hundred kilometres through Russia. You will do as you're told - pronto!"

For the Joneses it's a chance to see the country as no tourist ever sees it. Real Russia, with its poverty and sacrifice, its dirt and corruption, its warmth and charm. In one dilapidated "hotel" JJ complains about the lack of water for showers (70 riders have drained the tanks). I tick him off hotly. "These people are piss-poor and they are doing their best," I say, "while we are RICH FOREIGNERS COMPLAINING." We kick the pipes, curse a bit, and they start working.

For most it's the chance to raise money for an important charity, celebrating its 50th anniversary, while having an incredible experience. But Doug and Dec have reasons closer to home. Doug's son is four years old and severely brain damaged, with the mental age of a baby. The NHS is not enough, for the family also need information and support. Dec's son is only mildly affected but he can see how much help is needed life long. As he speaks to us on the final night, his love and sincerity make us weep. Here, then, is the answer to critics who allege we are getting holidays on the cheap while claiming to be doing it for charity. This ride is no picnic, and the total we've raised is £235,000, with many participants (including us) paying the holiday costs from their own pockets.

At 3 in the morning, after over 400 km, I am feeling seriously grotty. We're in the Yaropolets mansion which belonged to Pushkin's in-laws and is now the holiday home for Moscow University. The wallpaper is stained, the ancient loos and showers are half a mile away, the dining room is stifling. Ah, these poor people, I think: they murdered or chased away the original owners and replaced them with - something worse. My asthma has flared up badly. I doze, and dream of my daughter as a small child, crying. When I awake the tears are on my own cheeks. I am not going to be able to ride next day, a 120km killer.

Not till later do we realise we've been overdoing the mosquito repellent which contains Deet. We've sprayed tents and bedrooms, rubbed it on every bit of exposed flesh; I've been breathing it in all week. A day in the van is a big help and I am joined by five others including two casualties who fall in the (welcome) rain. JJ is nowhere to be seen. In my absence, he's gone up to the front, and belts along behind Ilya I and II the whole day. In the evening he's hyper, a dog with two tails. Even he needed to prove something, and now he has.

The final day dawns. "Only 70 k today, a doddle," says JJ. Then, "My God, did I really say that?" We don our blue Scope tee-shirts; we have to stay together on the busy route into Moscow. The police have been escorting us for two days. The first officers took one look at Big Jon and challenged him to arm wrestle. Villagers assembled to watch as he was beaten soundly. Perhaps if he'd won, we'd have been arrested…? But now it's Saturday; city streets are crammed with belching buses and cars. It's not only Rebekah who is terrified, we all are, but everyone is cycling, even the injured in their bandages. We stream under the ringroad, whooping like banshees. We get faster and faster; cars honk, drivers wave. We're streaking into the main shopping boulevard. The police have closed the streets and wave us through traffic lights; crowds have gathered and are cheering us! Suddenly we turn a corner, and it's uphill across punishing cobbles, and we're here.. in Red Square, with champagne and hugs and tears in the shadow of St Basil's cathedral and the Kremlin. Everyone has made it. Everyone.

That night, at the celebration dinner, Big Jon Summers at last takes off his shirt and gives us the haka. And it is awesome.


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