It's six in the evening and I've already ridden 80 miles off-road, over rocks and through streams. I've just been drenched by a cloudburst and terrified by the proximity of the lightning overhead. Right now, instead of riding, I'm hauling my bike, laden for four weeks of riding, up a cliff made of loose shale and even looser tree roots. One wrong move, it seems, and I or my bike – or both – could well disappear into the river below.

What's more, even if I can't see them, I know I'm surrounded by all sorts of dangerous animals: bears, both brown and black, mountain lions, maybe even wolves. After all, I'm on my own in the middle of one of North America's most pristine wildernesses – the upper Flathead valley in Canada has been described as the continent's Serengeti, such is its abundance of wildlife. Still to come is another nine-mile climb through more grizzly country, followed by a breakneck descent at the end of which comes my reward – 13 miles of tarmac to the next town with food and a bed for the night. That's if I make it before nightfall…

Welcome to the world's toughest bike race. The Tour de France may grab all the headlines, but cyclists with a sense of adventure are increasingly aware that the real – if very different – challenge is provided by the Tour Divide: 2,745 miles of off-road riding from Banff in Canada, down the spine of the Rocky Mountains, to the border with Mexico. Not only is it 500 miles longer than the Tour de France, the race throws up other obstacles that Lance Armstrong is unlikely to encounter – waist-deep snow, ankle-deep mud, temperatures below freezing in the mountains and above 100F in the New Mexico desert.

Then there's the fact the race is self-supported. There are no team cars, bike mechanics or masseurs to help you at the end of each 100+ mile day. Prize money is the same as the entry fee – nothing. You are permitted to take advantage of the services available in the few towns you pass en route, but that's it. Even then, the pickings can be slim. Ovando, in Montana, is typical. The sign you pass as you ride into the town reads: Population – "about 50"; Dogs – "more than 100". Of 43 starters this year, only 16 completed the race. The winning time was 17 days 23 hours, 45 minutes – I trailed across the finish line 10 days later.

But racing the Tour Divide is only one way to explore the Rockies by bike. In fact, the race follows the Great Divide Mountain Bike Route, the pre-eminent off road cycling trail in North America. The GDMBR was conceived in the early 1990s as a means for devotees of the then relatively new hobby of mountain bike touring to explore the Wild West.

The result is a route that is designed for its "cycleability", which is a large part of its appeal. I've been a road cyclist for a long time, but I only bought my first mountain bike last year, so technical riding – otherwise known as falling off – is not on my agenda. Fortunately, four-fifths of the route is on dirt or gravel roads maintained by various government agencies, with a further 10% on paved roads. Only the final 10% is on trails, and most of that is rideable for even heavily laden tourists. Better still, for those without a spare month to complete the whole route, it can easily be broken into shorter sections – my recommendations would be to try the 270 miles from Colorado's Del Norte to Cuba, New Mexico, which has the route's highest passes, the 135 miles from Rawlins in Wyoming to Kremmling in Colorado, or the 200 miles from Swan Lake to Helena in Montana.

It's also a route that meanders through some of the most evocative locations in the history of the westward expansion of the US and its development as a nation, as well as some of the most beautiful. From its beginnings in the rugged mountains of Canada, the GDMBR next crosses into the US in Montana. The mountains here are far from the highest on the route, but the sense of wilderness and isolation is perhaps at its most powerful. With what feels like 50 miles of forest in every direction (and bears, real or imaginary, behind every tree), it's not difficult to picture yourself riding through terrain that's changed little since the original homesteaders passed through a hundred or more years ago.

Until you hit the mining legacy of mid-state, that is. Gold, silver and a variety of other minerals – some precious, some practical – inspired a late-19th-century boom that shaped the landscape, often literally: state-capital Helena, with its charming, turn-of-the century Victorian architecture, was once home to the greatest concentration of millionaires in the entire US; in Butte, nothing can obscure the remains of Berkeley Pit, once dubbed "the richest hill on earth" thanks to the copper it produced but which is now no more than a heavily contaminated, oozing sore.

After passing briefly through Idaho, the next state is Wyoming where the route's landscape contrasts are at their most extreme. First up are the Tetons, the youngest and therefore pointiest mountains of the entire trip. Then comes the Great Divide Basin, a 4,000 square-mile plateau of desert scrub and grassland.

I spend a night camped at one of the few springs in the area with a genuine cowboy, living with only two horses and a rifle for company in a wagon that could come straight from the 1800s were it not for the pneumatic tyres. I may be in a bike race, but the offer of whiskey as a night cap and fresh coffee the next morning – brewed on a stove burning dried cow dung – are highlights of the trip.

Next comes Colorado, where the wealth of the inhabitants seems to increase with the altitude (up to a breath-shortening 3,360 metres at Indiana Pass). The state is now, with good reason, sold as a tourist destination without peer for those who love the great outdoors.

And country and western karaoke. Eating in the only bar in the only town for 50 miles while the locals are enjoying their weekly sing-along – with your participation a pre-requisite for receiving dinner – is not easily forgotten.

More than three weeks after my soft-shale-shuffle in Canada, I finally arrive in New Mexico's Chihuahuan desert. The last challenge is a 120-mile ride to the desolate border post of Antelope Wells. The sun is intense, the heat extreme. Yet the aches and pains of the day – and of nearly four weeks' cycling – disappear in an instant when the Mexican frontier is reached. Whether it's at the end of the Tour Divide race, or more laid-back touring holiday, a scratchy collection of concrete buildings and stoney-faced border guards will never seem more welcoming. The only disappointment is that few other adventures will ever seem quite as adventurous again.

• Paul Howard's book of his adventure – "Two Wheels on my Wagon" – will be published by Mainstream Publishing next spring.

Essentials

Route information for the Great Divide Mountain Bike Route, including maps , is available from the Adventure Cycling Association (adventurecycling.org). There is also a guide book – Cycling the Great Divide, by Michael McCoy – that covers the route in 62 days. Information about the Tour Divide race can be found at tourdivide.org.

British Airways (0844 4930787; ba.com) flies from Heathrow to Calgary from £526 return, and to Phoenix from £407 return. An open-jaw into Calgary and back from Phoenix costs from £502.