Things develop with tremendous speed, and no one can predict where they will be next year and why

- Che Guevara

The adventures you can have in Latin America depend largely on how much money and time you have. Some people spend their trips cocooned in holiday resorts; others toss their guidebooks out of the windows, relying on luck to direct their steps. Days can be dull and repetitive or exciting and enlightening, while unexpected challenges provoke bouts of introspection.

More days to add to the diary. Full of inner life and nothing more. A collection of failures of every kind and unchanging sources of hope.

- Che Guevara, January 1954

The soul of the Andes oozes from the cracks between the cobblestones of Puno, on the shore of Lake Titicaca in southern Peru, where the thin air turns life into a slow-motion battle against fatigue. The land is dry and brittle, and llamas graze in the fields while ancient, weather-beaten faces pause to stare, mute as the rocks around them.

These were my thoughts as I passed through Puno last year, half a century after Che Guevara, who lost an argument with customs officials there and had two "red" books confiscated. The pace of life appeared to have changed little in the intervening years.

Two separate wills moving out through the American continent, not knowing the exact aim of their quest nor in which direction lies their objective

- Guevara, aged 25, as he began his journey from Argentina to Mexico

In 15 years of travel in Latin America, the monumental figure of Che Guevara has been a constant presence, observing me from T-shirts, billboards, and books, and, of course, coming from the mouths of workers and peasants everywhere. "If only Che were was here" is a frequent refrain, as if one powerful individual could overturn the immense machine of global capitalism today. Once upon a time, the legendary Che was still only Ernesto Guevara de la Serna, a footloose medical graduate who set out in July 1953 with his friend Carlos Ferrer, looking for adventure, sex and the meaning of life.

Guevara found all three, and we can now read his thoughts from the period (1953-1956) thanks to Harvill Press, which has published Back On The Road, a selection of letters and diary entries from his decisive second journey through the Americas. This was the fateful trip that had its climax in a meeting between Guevara and Fidel Castro, in Mexico City in 1955, a revolutionary spark that changed the course of Latin American history.

The surprise in the book is the sense Guevara's writings reveal of a carefree youth with a conscience who might easily have continued his travels to Europe or Venezuela had it not been for a handful of fortuitous encounters.

My own wanderings through Latin America have taken me to many of Guevara's stop-off points. I completed the same trip from Buenos Aires to La Quiaca, on the Argentine border, continued into Bolivia and embarked on the trip from La Paz to Puno.

"It's all been great fun here with shooting, bombing, speeches and other touches that have broken the monotony in which I was living," said Guevara in Guatemala City in 1954. His stay coincided with the CIA coup that ended democracy and opened the door to 50 years of genocide, which left 200,000 dead. As a newly qualified doctor, Guevara volunteered to join a medical brigade to help victims of civilian bombing. He was mostly broke, relying on odd jobs to get by.

I assumed that father considered me tough enough to endure whatever came my way, but if you prefer fairy tales, I'll make up some very nice ones

- In a letter to his mother, 1954

This is important advice to the prospective Latin American adventurer, as tales of illnesses and war take years off the lives of those at home. Guevara's radical views attracted the attention of officials in several countries, resulting in brief spells behind bars.

In 1955Guevara was poised to make it in Latin America, working as a correspondent for Agencia Latina, an Argentinian news agency, freelancing as a photographer and working on a book to be called The Function Of The Doctor In Latin America.

If I ever detect in myself that the sacred flame has given way to a timid votive flicker, the least I can then do is vomit over my own shit

- Che Guevara, 1956

By the time he met exiled Cuban revolutionary Fidel Castro, the die was cast, and training began for the revolution ahead. Guevara's commitment to social justice and revolutionary change marked his short, marvellous life and his tragic, untimely death.

The Guatemalan coup experience taught Guevara, crucially, that the US would never allow an independent sovereign state to run its own affairs, a crucial lesson for the years ahead. He also saw that "a people in arms is an invincible power", lamenting the lack of popular resistance to the Central American coup.

Back On The Road is a wonderful glimpse into the maturing mind of a great man and a vital companion to the previous Che diaries that covered his first trip through Latin America and his post-Cuba experiences in Africa and Bolivia.

Back On The Road: A Journey To Central America is published by Harvill Press (£12 in UK)