It's unmistakable, the kind of glazed expression that comes across the face of someone on the outer edge of exhaustion. You can ask them the oddest question and still get, after a vacant pause, a slurred answer that is always "yes".

That's the way many of us felt this time last week as we staggered to the bottom of Scafell Pike, the fourth mountain in the Five Peaks challenge our team of 12 was undertaking for charity. If any of us had been composed enough, the grim truth would have sunk in - that we weren't going to climb the five highest peaks in each part of these islands in the target time of 24 hours.

Sure, we had two helicopters, but they could only fly by day. Now we sat watching the last light disappearing from this beautiful valley in the Lake District, and with it our chances of making it by air to Scotland to complete the challenge were being extinguished.

All sense of urgency gone, most of us remained slumped around the choppers, staring listlessly at the dusk and picking at a plate of stew. Months of training, days of meticulous packing and preparations, so many favours called in . . . all gone to waste.

Well, not quite. There was still the £60,000 or so raised from corporate sponsors for the homeless charity, Focus Ireland. And, ultimately, seven of the team did manage to travel north, first by helicopter to Glasgow, then in a madcap three-hour overnight taxi-ride, and complete the climb of Ben Nevis a painful but not-half-respectable 29 hours and 19 minutes after we first started.

For the record, the challenge started at 6 a.m. at the foot of a misty Carauntoohill (1,040 metres), before moving on to Slieve Donard (850 metres) in the Mournes. An hour-long flight across the Irish Sea took us to Snowdon in Wales (1,085 metres) by mid-afternoon, and Scafell Pike (978 metres) and Ben Nevis, by far the highest mountain at 1,344 metres, completed the challenge. At each mountain, we carefully recorded our ascents and descents, to comply with the exacting demands of the Guinness Book of Records. Logbooks were signed, photographs taken and the whole event was captured on film by a fly-on-the-wall documentary team. The backup team supplied food and drinks, Deep Heat and Red Bull, and lots of encouragement.

I never knew you could experience so many emotions in a single day. From elation, on rattling up the first peak, to anger on finding that the helicopter that was supposed to pick us up from the bottom had failed to arrive. It did come eventually, once the mist in Shannon had cleared, but we were never to make up the lost time.

As the day wore on, the group's mood turned quiet and even morose. We turned to glucose tablets and isotonic drinks to lift our energy levels, and knee-pads and ankle bandages to keep disintegrating body parts together. We watched the rest of the world sit in the park, swim in the sea, walk the dog, and wondered what madness we were embarked upon. Never again, we said at the end.

Now I'm not so sure. The target of 24 hours for the five peaks is still there (last month, a twoman team completed the challenge in just under this time, but they used car transport in addition to the helicopters in several places). But this doesn't mean that I don't have some doubts about the whole enterprise.

For a start, the helicopters that were the source of our undoing cost £25,000 to hire, thereby taking a sizeable bite out of the amount raised for charity. At times during our marathon, too, I wondered about the rush and fuss of it all. Mountains should be places of calm and tranquillity, but here were we puffing up the slopes at turbo speed then running off down from the tops without so much as a peek at the view. For many of us, the top of a mountain was a place with a good telephone signal; one team member even left his mobile on the top of Carauntoohill.

The Irish mountains mightn't be the highest in the world, but they are still havens of isolation. Yet increasingly, they are being turned into urban playgrounds; look at the invasion of the Wicklow hills each weekend by scrambler and quad bikes, four-wheel drives, and even huge walking parties which would lose their way instantly without their leader. There's a railway up to the top of Snowdon in Wales; how long is it before someone suggests putting one up Carauntoohill?

I know from the massive reaction to our challenge that it strikes a chord among people. There's the element of controlled risk, plus the chance of achieving a task that is neatly defined. Important, too, in the ever-so-busy world of the Celtic Tiger, that it doesn't take too much time.

Small wonder, then, that the challenge has spawned a number of related challenges, such as an event in which business people are being invited to contribute to charity while climbing the highest peaks in each of the four provinces.

Perhaps all of us undertaking such endeavours should first be required to ponder the lines of the poet: "They gazed and gazed but little thought/ What wealth to them the show had brought".