One year ago I rode out of Lima into the grey desert. Two years ago I was seeking shade in fiery Botswana beneath a baobab tree's spindly limbs. And three years ago was where it all began, waving goodbye to my family at home in Yorkshire. I rode out one midsummer morning with the hopelessly unrealistic dream of cycling round the world. Through my journey I aimed to promote the work of Hope and Homes for Children, an organisation committed and passionate in its work with orphans of war and AIDS. To my great surprise I am still going, and I sit today in northern Alaska with four continents now lying in my tyre tracks.

The last 12 months started and ended with reckless detours. On an ill-chosen "scenic route" through northern Peru I had pushed the bike uphill along a deserted track for some days when I saw a battered little bus approach; frantic Indians shouting and waving told me that they had just been held up at gunpoint. The driver accelerated away and I was left in silence with the unpleasant prospect of armed bandits up ahead. I stopped at every curve to crawl forwards and scout the land, leaping into ditches at the slightest sound and cycled through a sleepless night. Never have I been so happy to see a main road again.

A man in a cafe in Ecuador looked me up and down and declared, "with a body like that you'll never make it." I entered Colombia in a state of terror, wondering about the fine line between bravery and stupidity and deciding that the line does not really matter. Colombia is certainly loco but, feeling very selfish towards my family, I decided that surely the land of Shakira couldn't be all bad. And immediately the people and the lust for life of Colombia entranced me. Beautiful Cartagena was the shining jewel of Colombia, my reward for having taken the risk. As the Poker beer labels say in Colombia- "Where your friends are: that's your home."

Between Colombia and Panama lies 50 miles of jungle, the notorious Darien Gap. My plan had always been to try and cross it, but this time discretion won the day: I hitched a ride around it on a yacht. We anchored in the San Blas islands to work on the boat for a few days. The Rugby World Cup final was on and there I was stuck on a tropical island with nothing but palm trees, coral reefs, a yacht, lobster for dinner (again) - and not a TV for hundreds of miles. I suppose paradise is open to interpretation.

Sailing through the Panama canal, I learned how to use a sextant in child-like wonder: to be able to pinpoint your exact location on Earth using the sun and a wristwatch is astounding. Christmas Eve and a storm arrived together, hitting us hard in the night. I clawed my way up to the bow to take in sail with the boat heeling hard and the salt spray stinging, holding on very tight and feeling very alive. The storm peaked at 56 knots of wind. It was a memorable Christmas Eve.

In mad Mexico City- the urban equivalent of "how many people can you fit in a phone box?" - my host had to leave town for a weekend so she compiled a list of her girl-friends' phone numbers, each of whom had been briefed to entertain me. Latin American hospitality will take some beating. But I also saw families foraging on rubbish dumps and then attended parties in lavish penthouse suites.

The cold nights in the Sonora desert of northern Mexico were beautiful, toasting tortillas on small campfires among tall cacti. I always enjoyed riding through Mexican villages. Dim stores that sell the same few things the third world over - dusty pasta, cans of sardines spotted with rust, colourful warm fizzy drinks and stale biscuits. In the plazas old men sat still in the white sunlight, their faces carved with shadows beneath wide-brimmed sombreros. Curious old ladies stared at me unblinking. Preconceptions are generally made to be broken, but I was delighted to see that, in the best tradition of Speedy Gonzalez, rural Mexican men really do wear very large sombreros and say "Andale! Andale!" a lot.

I was sad to be leaving Latin America behind: I would miss the tightness and importance of families, the huge meals and the warmth of the people. But after being the odd one out for so long, I was excited to be returning to speaking English, looking anonymously like most other people, and the general ease of life in the United States. I felt like I was going on holiday.

At the border I had to go into a special room to explain why I had visas for lots of axis-of-evil countries in my passport. I was interviewed twice and paid a record $100 to get into the US. It seems in this era that even if "you're with us", you still might be "against us".

But they let me in and as I rode, mesmerised at being back in the first world the words "Large and in charge" scrolled across a massive billboard. A fitting slogan for the US, perhaps, but in fact just an advert for McDonald's. For me, seeing now with new eyes after two years in the developing world, the US was incredible. I went immediately to a supermarket to gawp in glee at the heaving shelves and the astonishing variety of breakfast cereal.

Los Angeles was the zenith of American hedonism and I wondered, how would it feel to be so rich and able to indulge in every spending whim? But then I realised that in most countries people gaze at me in fascination, at my own unattainable wealth, at my opportunities and my freedom. In most of the world I am a man on a fully loaded bicycle so I am very rich. In the US I was a man on a fully loaded bicycle so I was very poor.

I arrived in shining white San Francisco, the epitome of the beat generation's road fever. And, boy, here I was at last too! Wheee! Riding my darned bike up them crazy steep streets you always see in the movies, with those wild old trams and the tracks burning silver-white in the sunshine and mesmeric dreadlocked girls looking and smiling real pretty.

There was a massive outpouring of Irish-ness for St Patrick's Day. It is certainly a bigger event in America than it is in Ireland. Enormous volumes of Guinness, rivers dyed green; a celebration of heritage. Even MTV proudly celebrated the occasion- with a stirring, skirling, nostalgic bagpipe rendition of ... Flower of Scotland.

I rode north at Easter through the Redwood forests, awesome groves of the tallest trees on the planet, filled with a heady silence fitting for such majestic things - some of whom have stood since Jesus. Yet in the US something as startling as a 2,000-year-old, 300ft tree is not enough and so one of the trees now has a huge hole chain-sawed through it so that you can drive your car through the tree. Such depths of entertainment delight me immensely.

My experience of the US was massively positive, finding a friendly, hospitable, religious country with vibes surprisingly not far removed from Arabic countries I know and respect. This is not the country currently being portrayed to the world by its leaders. I headed on into Canada and wilderness beautiful and vast.

One of the worst fire seasons in history hung over the Yukon Territory like a pall. The Top of the World Highway was closed due to smoke and I could ride no further. Fortunately a friend joined me for this stretch, so we were able to pile up a 17ft canoe ridiculously high with our bikes, bags and two weeks' supply of food and push off towards the next road, 500 miles away, hoping that we would quickly figure out how to canoe.

I relished the new challenge and the ever-changing Yukon River; at times jade green and steady with strange boils of water rising and swirling. At times, a sliding mirror. At times shallow and clear so you could look down and watch the pebbles rush past, colourful time capsules, colossally old. We camped on islands, belted out Robert Service's virile poems, failed to catch salmon and watched moose and bear and imperious bald eagles.

But then we came upon the Five Finger Rapids which pessimists had long since declared to be our nemesis. We so nearly made it through on sheer luck and enthusiasm, but the canoe was too heavily laden and at each wave we shipped more and more water until, with a little sigh of apology, the canoe rolled, flipped and dumped us into the cold river. The river was fast and the canoe incredibly heavy. We feared she would sink. It would not have been ideal to lose my entire worldly possessions several days' hike from the nearest road. We whizzed helplessly downstream. The situation was just about on the cusp of turning from amusing into serious when we managed to haul ourselves into an eddy and get to shore. I have never known such uncontrollable shivering. Everything was soaked but we had, sort of, survived and we were jubilant. We paraphrased Lance Armstrong as our explanation for this foolish stunt of reckless, testosterone bravado, "50% was for the adventure, and 50% was for those who never believed."

Afterwards I continued north on the bike to Alaska, the end of my fourth continent and 44th country. Only now am I starting to let myself believe that I may actually be able to finish this journey. After 28,000 miles I now only have to cross to Siberia and ride across Eurasia! I am homeward bound at last.

· See Alastair's website at roundtheworldbybike.com for more information.

· If you feel moved to donate, please send cheques to:
Hope and Homes for Children
East Clyffe
Salisbury
Wiltshire
SP3 4LZ
or donate online at hopeandhomes.org