Each morning between 7am and 8am, fashion designer Julien Macdonald can be found on his bicycle, going at high speed around Hyde Park.

Sleek and slender, Macdonald loves the furtive, anonymous aspect of this exercise "because I'm in a helmet and glasses so they never know it's me, which is a great thing because otherwise I'd never get round the park".

In my mind's eye I picture Macdonald cycling while wearing sequins - perhaps because when we last saw him he was spinning around the dance floor as camp as Christmas on last year's Strictly Come Dancing. Or maybe it is because his dazzling dresses - usually found clothing such celebrities as Kylie Minogue, Kim Kardashian and Beyonce in a barely-there manner - are his signature trademark. Glamour is Macdonald's passion, his currency. Yet for five days in October, the former protege of Chanel's Karl Lagerfeld will ditch his design studio in favour of a more worthy cause, cycling 280 miles around Madagascar for the charity Enable Scotland, which is celebrating its 60th anniversary this year. The challenge - which will see him join 12 other cyclists, heading from the capital city of Antananarivo through villages, tropical forest and paddy fields before culminating in the coastal town of Mahavelona - aims to raise more than £100,000.

Such is the reason for the daily cycling training, of course; although the physical benefits of the Strictly Come Dancing ballroom can still be felt. "Thank God for my Strictly training," he says in his wonderful Welsh drawl. "Last year I was dancing around dance floors with a crystal ball for eight hours a day and this year I'm cycling up mountains in the wild bush for eight hours a day."

Where Macdonald finds the time I don't know. He has his fingers in many pies, from designing his own catwalk collection to his fashion and home ranges (Star by Julien Macdonald and Diamond by Julien Macdonald) for Debenhams - not to mention designing yogurt pots for Muller (having been roped into it by his friend Nicole Scherzinger, a spokeswoman for the brand) - it is a wonder the 43-year-old has time for charity work at all, especially ones that are so physically demanding (he climbed Mount Kilimanjaro for children's charity Chance UK in 2011). But then Macdonald seems unfazed by challenges; indeed he appears to thrive on embracing them.

"I was brought up in quite a working-class background," says Macdonald, whose father came from Inverness-shire. "People like myself, they never really did anything. My sisters never went to university, not many people left the valley where I lived and we lived quite an insular kind of life. I think over the years things have changed and people want to do different things, so when I get a chance, I always want to do those things. And I want to inspire others to do things because if other people see you doing them it's motivational."

He speaks specifically in regards to the children back in his hometown of Merthyr Tydfil, who he hopes will see him as "somebody they can relate to, who went to their own school" and be inspired also. "My parents were factory workers so I always go back to the school I went to and say, 'Come on, you guys can do it. You can be great doctors, you can be fashion designers, you can fly to the moon. You've just got to believe in yourself.'"

Macdonald is certainly a worthy role model. Having done a one-year foundation course at Cardiff Art College, followed by a degree course in fashion textiles in Brighton and a MA at the Royal College of Art, he soon caught the eye of Lagerfeld after the success of his 1996 degree show. The impresario appointed him head designer of knitwear for Chanel and, even today, one of Macdonald's career highlights - other than receiving his OBE a decade ago - was the moment Lagerfeld brought him down the catwalk and introduced him in celebration of all the work Macdonald had done for him.

After being named British Fashion Designer of the Year in 2001, Macdonald was appointed as successor to Alexander McQueen as creative director of French fashion house Givenchy. The appointment lasted two years and, from the start, had an element of controversy to it as many in the establishment viewed him - then dubbed the "Welsh Donatella Versace" - as a strange fit for the a brand most famous for dressing Audrey Hepburn. Although he has insisted he made money for the brand, some of his collections failed to win critical approval and when his tenure ended Macdonald returned to solely working on his own line before launching his more affordable lines at Debenhams in 2004.

But ask Macdonald if he has had any career lows and he refuses to admit any. "Do you know what? I couldn't say that I've had a career low," he says. "Everything's been a high. The lows can be when there are no clothes the day before the show, or shoes not arriving on time on the day." As such, the best advice he has ever received, he says, came from Lagerfeld, who told him: "Never look back, just look forward. Forget about the past because it is the future that is important."

Maybe it is all a confident facade, but Macdonald doesn't appear to get easily stressed. For example, although his cycling challenge is looming on the horizon, when we speak towards the end of August, London Fashion Week is just around the corner, and Macdonald says there isn't a single dress ready from his collection. Still, he seems unperturbed, even though he adds it will be a "big show" for him. "I'm like, 'When will all this happen?' But it always happens a few weeks before the show."

From the outside, it seems taking things as they come is Macdonald's modus operandi - and perhaps the only way he can operate when it comes to juggling so many balls in the air. He refers to it as being on the "fashion train".

"And that train doesn't stop. I just go for it," he says, "and it's always high speed. If you think about it or stop, it's hard to stop, so you just keep on going." But given fashion's relentless schedule, one that constantly trains the eyes of the industry on him, and him alone, does he not feel the pressure? "Not really," Macdonald replies. After all, he's on that train, he says.

I get the impression he tries not to think too far in advance so as not to be overwhelmed, and it's a theory he backs up. "When anybody needs me, I just have to go. And I don't look at my diary. I just wake up in the morning and go, 'OK, what am I doing?' The night before, I look where I'm going, what I'm doing and I get on with it. If you think about things and analyse it, you'll just get stressed out. And I enjoy doing it - whether I'm working on my Debenhams project or my own main line or whether I've got Kylie coming into the studio. Whatever it is, I deal with it that day."

Macdonald loves dressing celebrities such as Minogue, who has worn his creations on the red carpet and on stage, once cutting a dress of his in half to appear alongside Justin Timberlake at the 2003 Brit awards. She is the perfect ambassador for his brand, as his dresses are sparkly, fun and a little cheeky. Seeing famous figures don his garments is a huge ego boost for Macdonald; however, the same applies, he insists, to the average woman out on a Saturday night.

Women, he says, will say to him, "'I'm wearing your dress tonight,' and I'll say, 'Yeah, I know, it's from Debenhams.' It's nice to smile and have a picture with somebody and I'm proud of that. Whether the dress is from Debenhams or Harrods, I'm happy to chat to everybody. Fashion should be for everyone."

But what is the appeal of glamour, which so many of his designs embody to a tee? "I like to celebrate women and I like women to look glamorous and to feel like a woman. No matter what shape or age you are, everybody deserves to [feel that way]. There's nothing better than getting ready for a Saturday night. You might've had a week from hell but you can be a superstar for the night. I like the transformation of a woman into someone glamorous and gorgeous, the sense of dressing up."

Given the diaphanous fabric and revealing style of many of Macdonald's designs - a prime example being the dress Ellie Goulding wore to perform on The X Factor last year, the same outfit Beyonce wore in promotional shots for her Mrs Carter tour - what does Macdonald make of the culture that sees Miley Cyrus and Lady Gaga appear in public wearing little more than their underwear? "There is a time and a place for everything and when you're a pop star, anything goes. You're not going to go to your local on a Saturday night in a bra. Enjoy fashion and make it work for you."

Macdonald insists he isn't a workaholic (to unwind he watches Coronation Street) although he works hard when he needs to, an ethic he says stems from his parents who "woke up early, worked all day and came home late at night. I've always been around that kind of mentality where you need to work to pay the rent, pay the mortgage."

At first, they didn't understand why Macdonald wanted to be a fashion designer, or why he wanted to leave Wales. "But now they're proud," he says. "My mother, my sisters - they wear all my clothes at Debenhams.

"Luckily for me I have a job that I love going to: it's fun, I meet different people, I travel the world. So it's like a dream job. But there are a lot of people out there who just work for the sake of working and don't necessarily enjoy it. But I decided at a young age that this is what I wanted to do and I was focused and grounded enough to be able to do it. I look forward to going to work every day; it's not a chore."

At the end of the day, he says, he is a normal guy who happens to be a fashion designer. And while he lives a life that encompasses glitz and glamour (as well as graft), Macdonald recognises there is something to be said for the quiet family life his mother and two sisters have in Wales. "I don't have that kind of life any more. I have a different kind of life but I don't have my family around me and I think family is one of the most important things in the world. I sacrifice certain things for other things and it's not always the best thing."

Given this - and Macdonald's star-studded client list and the world of fashion in which he operates - I wonder how he manages or tries to stay down to earth. "Never forget where you come from," he answers, acknowledging the "family values" his parents taught him. "It's just about being a nice, grounded person."

As such, Macdonald views himself as someone who, although from a humble background, simply worked hard and made it in the tough fashion industry. "I've made a career for myself through determination. And you've always got to be nice to people because sometimes those young people you meet turn out to be fashion editors of magazines one day. All through the years, I've been successful at what I've done, I've worked hard and I've dressed the biggest celebrities in the world."

Despite Macdonald's apparently unwavering confidence and occasionally shameless name-dropping, the Welsh native seems a genuinely nice man. This, perhaps more than anything, is why he is so keen to give back via charities such as Enable, who help children and adults with learning disabilities. Without elaborating, Macdonald hints that he has family members who are directly affected themselves. "I think we all have family members who have learning disabilities and problems with their lives. We don't all live a perfect life."

The Madagascar opportunity came about after Macdonald hosted a luncheon for the charity in Edinburgh. The women involved told him about the challenge, assuming he would never do it, but something piqued the designer's interest. "I said, 'I'm quite interested in that. I like challenges.' They said, 'Do you cycle?' and I said, 'No. How hard is it?'"

In fact, the itinerary will be gruelling, with early starts and cycling on terrain of varying difficulty. So for the foreseeable future - as well as appearing on Strictly Come Dancing spin-off series It Takes Two alongside host Zoe Ball this autumn as official fashion commentator - the impending charity challenge will be Macdonald's main focus. That is, after he gets London Fashion Week out of the way.

"So I'm just going to get on with it. I can ride a bike for a start and it's a chance of a lifetime. To go to such an incredible place, it will be inspirational for my collections - the people, the plants, the flowers, the animals. And on a personal level, it's a chance to bring awareness to the charity, raise money and change somebody's life." n

JulienMacdonald's JustGiving page can be found at justgiving.com/julienmacdonaldobe. You can also sponsor him by texting ESMC60 £10 to 10070.