Cycling: Saddle up for a spin cycle guaranteed to put your body through the wringer

Tired of your Sunday ride? Then try pedalling 100km of off-road pain. Robin Barton joins a new breed of spokesmen

Marathon mountain bike races have been around for several years, but this year they have not only gained official sanction - Britain's first national marathon championship took place on 17 July - but also now outnumber all other entries in mountain biking's calendar.Having signed up for the Builth Wells Merida 100 Marathon in July, probably the largest ride of its kind in Britain, I first met Beckinsale in May on Bristol's Timberland Trail to pick up a few tips. Three months, he tells me, is enough training time to be able to ride for four-and-a-half hours: "And on race day you'll surprise yourself with what your body can do; adrenalin is amazing stuff," he adds.

Beckinsale cycles 400 to 500 miles a week on the road in winter, and does 80 per cent of his training on a road bike. His season got off to a flying start with back-to-back wins in the National Points Series (NPS), but he has had only two weekends off since March.

He started hard racing in mid-March, and was feeling strong by April. By the end of May he had to surf a wave of good form for eight important races: "Take a week off and the first thing that'll happen is that you'll get ill." Between mid-March and mid-September he aims for three peaks in performance.

Fortunately I have to find just one peak and, Beckinsale assures me, the Merida Marathons are actually closer to 90km than 100km. He will be riding the Builth Wells Marathon, but will treat it as a training session; he is capable of finishing a 100km marathon in 3hr 30min. "The problem with being a professional," he says, "is that there are a lot of guys who'd like to say they beat you. But you can always put the hammer down."

So how should I prepare for a 100km mountain bike marathon? "Add about half an hour each weekend to your rides," advises Beckinsale. "After six weeks you will be able to do a four-hour ride at your own pace. Try to stay flexible, and don't lose motivation if you break your schedule - just do twice as many miles the next weekend," he jokes. I will need to ride twice a week, cut down on alcohol, tea and coffee, drink two litres of water daily and stretch after every ride.

Beckinsale also recommends building core stability in the abdomen and back - power is channelled by the back and hips, and, he warns, "a bad back will come out on an enduro ride".

Seven weeks later, I put Beckinsale's pre-race plan into action: "Forty-eight hours before the event, fill up with carbs. Graze during the preceding days so you don't use your body's glycogen stores. The day before, have a substantial evening meal, a snack before bed and a big breakfast."

With a hydration pack filled with three litres of water and any remaining space crammed with energy bars, spare inner tubes and a tool kit, I join 1,150 other bikers on the start line. Those who camped overnight in Builth Wells say there was a torrential late-night rainstorm, but the day has dawned bright and clear.

The Builth Wells event is the centrepiece of the five-race Merida Marathon 100 series. The whole loop is waymarked, with shorter options available, and there are food and bike servicing stations at intervals. "Marathons are more accessible than other forms of racing," says Michael Wilkens, the co-organiser. "Cross-country racing used to be about heads-down aggression, but marathons allow riders to trundle along with friends or treat it as a serious race."

There is certainly no rush as we set off through the narrow Welsh lanes and up into the countryside. "Are you a closet roadie?" someone shouts as I pass them going up a hill. I will admit to being an avid road-cycling enthusiast, but it helps being on the bike Beckinsale's sponsors, Scott, have loaned me: a full-suspension, carbon-framed Genius MC20 designed for marathon rides. Aside from the low weight, what I like most about it is that with a flick of a switch on the handlebar, I can turn the rear suspension to a low-travel, high-traction setting for climbing.

The first part of the ride is a straightforward slog up wide, hilltop bridleways, but after about 20km I tear a muscle in my calf and the ride becomes less about beating people than reaching the finish. "Listen to your body" is another Beckin-sale tip: mine is telling me to hitch a lift back to Builth. And the finish suddenly seems even farther away when the trail enters the woods and becomes single-track: the heavy rain has turned what should be a blast through the woods into an impassable, insect-infested bog. Riders are having to walk several kilometres, which is going to add hours to our times. Hobbling along, pushing £2,500 of mud-caked bike, I vow never to do another mountain bike marathon again.

It's been a miserable, frustrating exper-ience, but already at the third feed station anecdotes are being shared. "Lofty, is that you?" one cyclist says to a mud-encrusted co-rider. "Have you been through that mudbath they call the single-track?"

"Dude, down one hill in the forest I was accelerating with both wheels locked!"

Four-and-half hours after the 10am start, Beckinsale crosses the line in second place. At one point he had been two minutes up on a local racer, but suffered a puncture after the final river crossing and couldn't catch the Welshman again. Three hours later and I finish, coming in about halfway down the field. It turns out that the course was 110km rather than 90km, and Beckinsale admits that it was the hardest Merida marathon he has done.

"It's very satisfying seeing people come in," says Wilkens. "At the Selkirk event last year there were grown men coming over the line in tears, saying that was the best thing they'd done on a bike. People do a mountain bike marathon for the same reason as non-runners enter the London Marathon: they're proving to themselves that they can do it."

First-timer Ruth Brooker found it as much a psychological challenge as a physical one, and we were both surprised at the lack of technically difficult sections. "It must be because the event attracts fitness cyclists," she thinks.

"We had positive feedback," says Wilkens, "but personally, I think it was too hard. It's not safe for people to be riding until 8.30pm then have to drive home. But, let's face it, a marathon is not supposed to be a walk in the park."

Two weeks later, Beckinsale's 36-year-old Scott team-mate, Nick Craig, wins Britain's first national marathon championship, completing the 100km race at Margam Park in 4hr 03min, although Beckinsale had sealed victory in the 2005 NPS cross-country series the previous day.

For details of the Merida Marathon 100 Series: www.mtb-marathon.co.uk. More information about Scott bikes: 01670 712 129, www.scottusa/com