For some people, it started with an alluring poster in the office corridor; for others, it was an advert in their local paper; for a few it was repeating the experience of a lifetime. Whatever the reason, sponsored treks are an increasingly popular way of combining trips to remote places with the chance to make a difference to other people's lives.

Destinations range from Tibet or Costa Rica to Namibia or the Sahara desert, and the causes vary just as widely, from learning disability support to cancer research to help for deaf children.

I chose to trek for Mencap - or rather, the trek chose me. Lurking next to the lift at work was a poster advertising the trek with a mesmerising picture of three granite peaks towering over a turquoise lake, and after one glance I knew subconsciously that I wanted to do it. I struggled with the idea for a while, but ultimately I knew I would give in and raise the £2,250 required to go trekking in Chilean Patagonia, at the very tip of the South American continent.

The concept is a heady mix of exotic location coupled with the opportunity to participate in a challenging event which requires preparation, logistical support and knowledge of the terrain. For the charity involved, sponsored treks are a good way to raise their profiles and tap into new sources of funding via previously unsolicited networks of family, friends, colleagues and local organisations. Treks can raise a substantial amount of money for the charity - on my trek, close to £150,000 was raised by about 100 participants (split into two groups trekking over a month). With half the money raised going towards the cost of the trip, the outlay for the charity is minimal. It's all down to the participants to start fundraising.

Initially, £2,250 seems such a huge amount of money that the task of raising it is insurmountable, and there are moments when you seriously question your sanity. There's definitely a genuine time commitment involved - be it through the organising of activities designed to raise money or just in terms of coming up with ideas on how to get that damn dough.

One simple method used by a fellow trekker was to ask 100 people he knew to give him £20 each - although that obviously involved knowing a lot of generous people. Another participant boasted of raising money through stripping, but when challenged to repeat the performance on the trip, flatly declined.

In the midst of all the excitement and as the money builds up in front of your eyes, it's also easy to lose sight of the training required for such challenges. It's emphasised in the literature you receive, but when you consider that you'll be travelling to a remote location (it took three full days in our case to reach our final destination), that you're likely to be jet-lagged and will not sleep very well in your strange and none too comfortable surroundings, the preparation you put in will prove life-saving.

As part of a group, you probably won't be able to walk at your own pace, which in itself is tiring, and becomes problematic when you are walking an average of 20-25 kms a day (the entire trek was 125 kms - about 100 miles - over five days). Because the group is by definition diverse - having drawn together people of all ages and background - various abilities and attitudes will be represented, from the die-hard trekker to the "I'm not on holiday to exhaust myself"-type rambler.

But the rough, rugged landscapes of the Torres del Paine National Park, reminiscent of the Scottish Highlands in some parts, the Swiss Alps in others, certainly help you to forget your sore knees, aching ankles and blistered feet, and a hot shower or the simplest meal seem like gifts from heaven.

On the final day of the trek we climbed towards what would be the culmination of our trip - the famous towers of the Paine Massif, the ones I had spotted on that poster many months before. It was the most difficult day of our trek - the walk started off up a long, unrelenting slope among rolling hills which took several hours to ascend, followed by a spell of clambering in the woods and then about an hour of scrambling over boulders to reach the top.

In my group, everyone gradually dropped out - victims of sore knees or sheer exhaustion - until only my sister and I were left with the medic. As we carried on up the long incline, the groups ahead of us encouraged us by singing to us over the walkie-talkie. Soon the sun was blazing down and we began to struggle, but when we got to the refugio at the halfway point the sight of the pretty chalet and bounding stream injected renewed energy into our tired limbs.

After a spell in the neighbouring woods, we reached the final stretch of moraine. The trail looked clear enough at the edge of the woods but it soon dwindled into nothing, until we were left clambering over rocks like overweight goats.

When I thought I had come to the end of my tether, a middle-aged Chilean woman sporting a stick stopped to spur me on - or, more accurately, to harangue me. She told me indignantly that if she could make it to the top, someone like me - half her age - surely could, and that I should enjoy this part of the walk while I could, because getting back down again was going to be far more difficult.

With these words of comfort ringing in my ears I forged on, feeling so completely exhausted that I doubted my ability to crawl up the final vertical metres to the summit. But Janice the medic (who had heroically relieved me of my pack) single-handedly dragged us - physically and emotionally - to the top.

Incredibly, when we got there, the three other groups were booming La Marseillaise at the top of their lungs in a surreal greeting, and as a token of their appreciation that two limping froggies had made it in spite of the odds.

And the feeling I experienced when I turned around and laid my eyes on those breathtaking peaks was like no other feeling in the world - an overwhelming mix of exhaustion, elation, amazement, achievement and gratitude. Once in a lifetime indeed.

• Mencap organises a range of adventurous activities which combine a personal challenge with the opportunity to raise funds for people with learning disabilities. To find out more, email events@mencap.org.uk or telephone 0845 9777 779 or go to the Mencap site.