Imagine you are Che Guevara during his last and fatal campagin in south-central Bolivia - a country rugged, ravinous, spiteful and sublime

Call this a pilgrimage and name it "In the footsteps of San Ernesto de la Higuera". This is a homage, by horse, foot, cuts and bruises through rural Bolivia to the man Graham Greene once described as being in "a state of permanent revolt" and Jean-Paul Sartre as the "most complete man of our age". It is also a voyage around a country of stunning natural beauty and irredeemable poverty.

Ernesto "Che" Guevara was my hero when I was 11, usurping Captain Scott and rivalling Christ. The Argentinian doctor, who became Castro's left- wing man and an international revolutionary, remains a hero of sorts: an avenging angel with clipped wings, flawed certainly, but all the more human for it. Rightly or wrongly, Che came to believe that the only way life would change for the better for the poor and oppressed people of South America and throughout the Third World (he fought in Africa, too) was through the barrel of a machine-gun. Say he was wrong (easy to do from the equilibrial position of an armchair in superstore Britain), but Che (a father of five) believed it was better to be Red and dead at 40 than carry the onus of having done nothing but talk about what is to be done into a complacent middle-age.

Now, try and imagine yourself as Che Guevara in south-central Bolivia, scene of his last and fatal campaign, in October 1967. The landscape is rugged and ravinous, spiteful and sublime in equal doses. The cold at night, summer or winter, can be marrow-chilling; if the sun passes behind a cloud at midday in summer, a chill will wrap around you like the talons of a ghost. The dense and thorny vegetation tears at bared skin. To hack through these barbed walls of unrelenting green, a machete is essential. Mosquitoes and other buzzing, winged and crawling nasties are inescapable in the region's profuse river valleys. They are all big and they all bite. You are also allergic to mosquito bites.

As the most wanted man south of the Tex-Mex border, you are surrounded by a US-trained counter-insurgency unit. They have been instructed by Washington to kill you after interrogation by CIA agents riding shotgun with them.

You can hardly breathe (you have suffered from chronic asthma since you were a toddler) and have run out of medicine. Your 39-year-old frame is also racked with malnutrition and rheumatoid arthritis. Your chestnut hair is shoulder-length and matted, your beard is long and streaked with dried mud. You look like a medieval mystic, a saint in the making, which is why, in those famous press photographs showing your corpse laid out on a farmyard slab in Vallegrande, you look, as John Berger said at the time, like Mantegna's Dead Christ.

Who would end up like this - exhausted, emaciated, tortured and riddled with bullets from a machine-gun that virtually severed one of your legs at the groin? But what things you saw, Major Guevara, both in your mind's eye - a vision of a brave and humanitarian future - and through those blue eyes. They penetrated this mountainous tract of land and marched imperiously south of Santa Cruz, which is bounded in the west by the exquisite colonial capital, Sucre, to the north by the small ragged towns of Vallegrande and Samaipata, and by Camiri in the south.

This was a dreadful place in1967, and remains today one of the poorest parts of the world. In fact, the Aymara and Quechua-speaking Indians (or campesinos) who eke out a living here are worse off than they were when Che took up their cause 30 years ago. The poker-faced Indians paid no attention to Che's guerrilla band, although you will still find photographs of the dead revolutionary pinned up in stone and mud huts in mountain villages, along with dolls representing the gods and goddesses of Earth, Luck and Coca.

Almost everyone chews coca leaves here. Chewing keeps hunger pains at bay, alleviates mountain sickness and paints rosy pictures in the unpredictable sky when the chicha (maize beer) runs out or when magic mushrooms and hallucinogenic cactus juice are unavailable. Along with the Bolivar, here coca is currency.

Ninety-seven per cent of people in the area live in "extreme poverty", according to a recent United Nations report, which states that "not even the poorest African states experience such general and extreme poverty". The evidence of your eyes agrees. Infant mortality runs to about one child in 10. Ten per cent of the population is involved in growing or processing cocaine for the Colombian drug market, despite a recent crackdown by the Bolivian government (60 per cent of Bolivia's foreign earnings come from the cocaine trade). One in two adults is unable to read. There are virtually no cars and anyway most of the roads here are unmetalled tracks, acrid and dusty in the dry season, for the most part unpassable when the rains fall.

There are road-blocks aplenty and poor, teenage soldiers galore in these vertiginous and poverty-stricken valleys - and policemen, both fake and real, who bother the few strangers for documents, but mostly for money. This is no place for any but the most bloody-minded tourist, although even here, in villages trapped between 5,000ft and 10,000ft above fast- flowing rocky rivers, there is the wistful balm of Bolivian music.

In Samaipata, a village taken and held for 24 hours by Che Guevara (6 July 1967, searching for insulin for his asthma), I tie up my grey-white horse. Her name is "the Bitch", because she causes trouble on the rackety ranch I rented her from in Jorochito, south of Santa Cruz, the provincial capital. I call her "sweetheart", which she is after several days of grooming and regular feeds.

I lean against the saddle and listen to the sad zampona (pipes) intoning a haunting mountain song. The Indian boy playing is thin and filthy. He is joined by his chums, a gang of young Incas for whom Santa Cruz, let alone La Paz, is the end of the world.

They add the sweet jangle of the charango (mandolin), the primal rhythmic thump of the bombo (leather drum), and the scaly rattling of the ch'ulla (a rattle made of shells and goat's bones strung together). It is one of those magical moments when the exhaustion, bug bites, sunburn, loneliness and rotten stomach are forgotten. This patch of Bolivia, as big as Scotland, but unmentioned in guides and poorly surveyed by map-makers, can be close to limbo, but when the music plays and the chicha is handed round, the sense of elation can reach as high as the peaks above you.

The villagers are cautious of foreigners and have every right to be. Foreigners include anyone from beyond their mountains. For four centuries foreigners have brought little else than death, disease and ruthless exploitation. To say you are English means nothing. The fact that you do not speak Aymara or Quechua marks you out as a potential danger. Police? Government spy? A hombre from the local drug barony?

No, a romantic fool on the trail of Che Guevara. Hardly a passport to a cosy chat by the fireside, although only the oldest villagers can recall the events of 1966-67 (a long time ago, when the average lifespan for men barely nudges 50) and then in contradictory and fictional detail. In any case, they know the military police have already stopped me for questioning. No outsider comes this way except on official, military or narcotics-related business, which are all much the same thing here.

So, I buy feed for the horse, trade cigars for an inexhaustible supply of mate de coca (the local "cuppa") and sleep under the stars. Tracking down Che's first command hut in a ranch above the Nancaguaza River, we see puma tracks (but no pumas), smell jaguar (and see one lazing above us in a tree), spot a Jeep (the only car in three days) throwing up a trail of dust on what must be the road from Muyupampa to Santa Cruz. But, as my maps are covered in squashed insects, berries and sweat, and bristle with such helpful hints as "approximate alignment", "data incomplete" and "possibly impassable", I get lost several times. Perhaps that Jeep is on another road altogether. Thank God for the compass.

There are times, especially when trying to find the river crossing where half of Che's band was ambushed on 31 August 1967, and again when looking for the Yuro ravine where he fought his last battle (17 men against thousands), that I lose track completely. I find it eventually, hungry and thirsty.

And yet, when you have managed to bruise a few ribs and feel hot, bothered, filthy and bitten, when there is another leach to burn off and your bowels have turned to sandpaper and jelly, you want to be anywhere but here. God knows why Che thought he could succeed from a base in these beetle- bombed ravines. Washington was snapping at his worn heels, but it was Bolivia itself, the indifference and fatalism of its mountain people and the impossibility of its secret landscape that, in the end, simply swallowed him.

Bolivia could swallow any one of us. Away from the few tourist trails, it is like a hungry anaconda. I am not surprised that Che lost and died here, but still surprised that he chose these desolate mountains and wretchedly poor and silent people as the raw material for a pan-American revolution. Jon Lee Anderson's biography - the first detailed and objective life of Che, to be published next spring - will tell us why.

Until then, I can only wonder that he survived as long as he did, surrounded by such sublime and monumental indifference.

How to get there: The closest airport is Santa Cruz, most easily reached from Britain via Miami, Rio de Janeiro or Buenos Aires. South American Experience (0171-976 5511) has a fare of pounds 641 on Aerolineas Argentinas. Journey Latin America (0181-747 8315) has 24-day tours of Bolivia, price pounds 1,540, with departures on 27 October and 15 December.

How to hire a horse: Jonathan Glancey paid $25 (about pounds 15) to rent a horse for a week, from a finca (ranch) outside the small town of Samaipata, in the foothills south of Santa Cruz. Hay and oats are widely available for sale at villages along the way.

How to navigate: The American military Tactical Pilotage Chart for Bolivia is the best map available. It costs pounds 7.50 from Stanfords, 12-14 Long Acre, London WC2E 9LP (0171-836 1321).