Webber, 36, is 184 centimetres (a smidge over six feet), which makes him something of a giant among his jockey-like rivals, but weighs just 74 kilograms.

Even so, the lanky and laconic Australian is still 10 kilograms heavier than his world champion Red Bull Racing teammate Sebastian Vettel, who is just 176 centimetres.

While the combined mass of an F1 car and driver can be no less than 640 kilograms, Webber is handicapped compared with Vettel because his weight sits higher.

The heavier ballast required to meet the minimum in the young German's machine is distributed more advantageously, lowering its centre of gravity fractionally. But it's enough to make a difference of hundredths of a second a lap, which is often the margin between positions at the front of the grid.

As well as mitigating his size, Webber's extra fitness wards off the several years of youth he concedes to most of his opponents.

Webber's body is sinewy and his face looks gaunt, but the appearance is deceptive as his strength and stamina are those of a world-class endurance athlete and few in F1 can match his extreme fitness.

Between the regimen set by his team's trainer and his own program, which includes running and cycling, he is tuned to a corporeal and mental peak for F1 racing, which is a hostile environment of debilitating heat and bruising physical forces.

But as fit as he is, even Webber admits that he will be underdone compared with the elite athlete competitors in his annual charity challenge in the wilds of Tasmania, beginning on Wednesday.

In its sixth running, the Swisse Mark Webber Tasmania Challenge is a five-day endurance test, with two-person teams contesting daily mountain biking, kayaking and trekking challenges over a 350-kilometre course through the Apple Isle's rugged north and north-east regions.

''I'd always like to be better prepared for it,'' Webber told Fairfax Media ''This season [a record 20 races] has been very long - getting all my excuses out early - but that's just how it is. Clearly, I'm not a full-time athlete - I'm a racing driver - but I like to go down there and do reasonably well. But I'll be open, I'm not where I used to be in terms of those three disciplines in years gone by because I have to prioritise on being fit for my job.'' Webber will fly to Launceston from Sao Paulo after the season-ending Brazilian Grand Prix, which will be run in the early hours of Monday morning Australian time, to make Wednesday's start, although he won't take part until Friday.

The Tasmania Challenge, which raises funds for the Mark Webber Foundation, finishes in Hobart on Sunday. All the proceeds go the foundation's nominated charities, the Leukaemia Foundation, Save The Tasmanian Devils Foundation and the Whitelion Foundation. The five events Webber has staged since 2003 have raised $1.5 million for good causes. The capacity field of 80 competitors, who have paid up to $10,900 each for the privilege of testing their physical and mental limits, is divided into three levels of performance - elite pairs, enthusiast pairs and enthusiast teams. For the first time, prizemoney is being awarded to the elite athletes, with the winners receiving $20,000.

Headlining the elite entrants are French adventure racing world champions Jacky Boisset and Mimi Guillot, who won the title in Tasmania last year.

Among the celebrity pairs are North Melbourne legend Glenn Archer and his former Kangaroos teammate Leigh Colbert, who teamed in last year's event.

Archer, competing for a third time, is adamant that mountain biking, kayaking and trekking through rugged terrain is tougher than the most rigorous AFL training.

''Adventure racing is long and gruelling, and probably hurts more mentally than AFL training,'' he said. ''It is considerably harder than AFL. It's probably the hardest thing I have ever done. We will be basically competing against ourselves. We are not at Mark Webber's level. He's a freak, unbelievable. He is the fittest guy I have ever met.''

Webber admires the willingness of the non-specialist athletes and amateurs to endure extreme hardship for charity. ''I still get a buzz out of people preparing for it and getting themselves through the event personally,'' he said. Webber is also proud of how much the Tasmania Challenge has raised to fund charitable projects such as the Leukaemia Foundation's ''Haven Of Hope'' apartments in Preston.

''It's well clear of a million dollars, so it's not what [billionaire philanthropist] Bill Gates is doing, but it's better than doing nothing,'' he said. ''We've built a few little units in Melbourne, which from some of the letters I've received have helped people that, clearly, have some adversity in their lives.''