About halfway up the hour-long first climb, it occurred to me that this might not turn out to be the roaring triumph I had hoped for. The plan had been simple: take two friends who are hard-core road riders - one of them a former London cycle courier, with a sleek Rapha wardrobe to prove it - transplant them on to bikes with suspension and knobbly tyres and convert them to the one true path that is mountain biking. In my wilder flights of imagination, I had pictured them being so smitten with the world of dirt that they would immediately burn their lycra and denounce leg shaving for ever.

Then came reality. Almost from the start, one of them was struggling on the sharp, slippery uphills that kick off the 15km Twrch Trail at Cwmcarn, near Newport in south Wales. After one tricky section, he threw down his gleaming new Trek full-suspension rig - the same rig he had been lovingly stroking in the car park - and growled: "First thing Monday morning, that bastard is going on eBay."

The Twrch Trail climb is a test for experienced riders, so it's fair to say I was chucking them in at the deep end, yet I was still surprised that a powerful, fit, committed cyclist could be thrown by the change of environment. I had assumed, perhaps naively, that all those hours in the saddle, the survival instincts of a battle-hardened bus dodger and the muscle memory from spinning countless circles on the road would be more than enough to carry him through the initial shock of not being on smooth asphalt. That's when it dawned on me that the differences between mountain biking and road riding may be more fundamental than my carefully constructed utopian vision had allowed for.

The biggest, of course, is the terrain. Riding offroad requires three-dimensional thinking. It's not enough just to be able to propel the bike forward; you also have to propel it over a surface that can change with every pedal stroke, from hard-packed dirt to soft loam to thick mud, any or all of which will be dotted liberally with (often concealed) nasties such as treacherous wet roots, tree stumps and hard, pointy rocks. On narrow sections of single track, with steep drops to the side, there is always the nagging thought that any mistake on the rocks and roots will have consequences. Exposure, the trail-builders call it.

There is also the technique issue. The latest bikes are sophisticated pieces of engineering, dripping with technology - gears, suspension and disc brakes - to take on ever tougher terrain. But having 27 gears is only useful if you know when to drop down through them to make it up a short, sharp climb, or how to use the drive-train to punch the bike over obstacles such as logs and rock steps. Suspension is great for soaking up the bumps, but it also helps to know how to use it to pump the trail for speed. Hydraulic discs provide reliable, instant stopping power but grab them at the wrong time and you could end up teeth down in the dirt.

And then there's the different physical challenge to maintain speed and line on constantly shifting terrain involves constantly shifting your body weight and moving the bike beneath your body. So balance and core strength are just as important as leg power and stamina.

Laid out on paper these all sound more than a touch daunting. What fool would go mountain biking when it's clearly such hard work? But these ingredients are exactly what make it such a brilliant sport: unpredictability, speed, nature and a constant element of danger. There is no better feeling than flowing fast down a twisty natural trail, just on the right side of control, feeling the bike float over obstacles and rail around corners.

This column rightly devotes a lot of space to the challenges of urban cycling, the daily encounters with psychotic drivers while avoiding ruthless bike thieves and trying to stay in one piece despite the efforts of moronic town planners. That's what I call hard work. If you want to return to the simple joy of cycling - the sort that comes from seeing a bike as a toy again, rather than a mode of transport - go off-road and get mucky.

Just ask my mate. We reached the top of that climb, stopped for some calories and then hit the first descent. Suddenly it all clicked into place and for the next couple of hours he threw himself and his bike at the trail, attacking climbs and charging at the descents, even the loose, off-camber, super-exposed sections that complete the loop. Back in the car park, the huge grin on his mud-caked face said it all: his bike won't be on eBay any time soon.