If you thought marathons were tough, think again. A new breed of extreme runner is spurning the 26 miles that did for the Ancient Greek messenger Pheidippides, opting instead to run anything up to four times that distance – across deserts, up mountains or through jungles.
Demand for ultramarathons, which include last weekend's Ultra-Trail du Tour du Mont Blanc in France, has surged in the past 12 months, with places filling up months before the event. The Marathon des Sables, a six-day 150-mile endurance test across the Sahara in Morocco, is booked up until 2014; it had four times as many UK entrants as places this year. The excessive distances are most popular with middle-aged men, who are swapping fast cars for trainers to deal with their mid-life crises, race organisers believe.
Marc Laithwaite, a sports scientist who started the Lakeland 100, a three-day 100km race across the Cumbrian fells three years ago, said people were opting for new experiences over setting new personal bests in shorter races. He compared demand for extreme events to the "marathon boom in the 1980s", adding that the Lakeland 100,with 500 entrants this year, would double in size by 2012.
Despite the physical demands of round-the-clock races, organisers say participants are not athletes but ordinary people looking for a challenge. "Our lives are getting softer. People don't go out and get muddy and dirty or use compasses or maps. They see running ultra-events as getting back to basics," said Ben Mason, who arranges races including next month's marathon-a-day 78-mile Atlantic Coast challenge for Votwo, the events and adventure company.
Rory Coleman, an ultra-running trainer who has completed 165 ultramarathons since taking up running 17 years ago aged 31, said the biggest proportion of competitors were accountants, then anyone working in IT. "They tend to be professional people who sit at their desk and wish they were doing something else."
Justin Bowyer, who tracks the growth in ultra-events for the website Running Monkey, added: "It's been called the new middle-age crisis. The greater distances tend to appeal to older people. You are only competing against yourself. There's an incredible camaraderie. It's a healthy and relatively cheap sport. You just need to stick on a pair of shoes and some shorts."
The most recent research by Mintel found that the number of adults running regularly hit 510,000 in 2008, almost twice as many as in 2006, which is translating into strong sales for sportswear companies. VF Corporation, the US sports giant, recently said sales of its North Face brand surged by double-digits in the second quarter to July. Subscribers to Runner's World , the specialist magazine, increased by 15 per cent to 96,352 last year, and the website Running Bug will have a separate section on ultra-running when it relaunches next month. Ultra-runners can travel all over the world to find races, which are springing up in the most inhospitable places, from the Canadian Rockies to the Amazon jungle. Or they can opt for one of more than 100 ultra-events being held this year in the UK.
Kerry McCarthy, senior writer at Runner's World , said people were running further because they were realising "their bodies can do more than they thought". He added: "[Ultra-events] sound substantially scarier than they are. You can usually take your time. Plus the training is not as arduous as you might think and often requires you to walk. You need to put in a lot of time and effort, but the training is less structured than for a marathon."
That isn't to downplay the impact on runners' bodies. A Swiss study found that endurance marathons caused a "statistically significant" decrease in skeletal muscle mass. Ultra-runners will have a resting heart rate up to 43 per cent lower than that of a non-runner and some 27 per cent less body fat. Their organs and tissues will also be able to absorb up to 50 per cent more oxygen.
There's also the potential for injury. Andrew Murray, the doctor behind Medical Marathon Services, said blisters and chafing were the most common injuries, followed by patellofemoral syndrome, which is pain at the front of the knee, and then Achilles tendonitis. "Some people will run through anything. One guy broke the main bone in his lower leg and continued to run for a further 40 miles. He was in plaster for eight months after," Dr Murray said.
The world's oldest ultramarathon is the Comrades Marathon, a 90km race held every year since 1921 in South Africa, except for a break during the Second World War. It was started by Vic Clapham, a First World War veteran, to commemorate South African soldiers killed during the war. More than 300,000 runners have completed the race, which must be finished within 12 hours. The oldest man to complete the Comrades, which takes runners along a route from the coastal city of Durban to Pietermaritzburg, the capital of Kwazulu-Natal province, is Wally Hayward – a South African Olympic runner – who managed it, aged 80, in 1989.
World's toughest race: 'Just 40 per cent manage to finish'
* The Badwater Ultramarathon covers 135 miles from California's Death Valley to Mt Whitney, in temperatures up to 55C. It starts 85m below sea level in the Badwater Basin, and ends at an elevation of 2,548m.
* The Ultra-Trail du Tour du Mont Blanc takes competitors through three countries – France, Italy and Switzerland – and crosses six major mountain passes, ascending a total height of 8,900m. Only 40per cent of entrants usually finish the event, which has a time cap of 46 hours.
* The 152-mile Spartathlon, which retraces the route historians believe Pheidippides took from Athens to Sparta in Greece, must be completed in a single stage within 36 hours.
* The Marathon des Sables, across Morocco's Sahara Desert, is a multi-stage event that requires runners to complete 150 miles over six days, including one stage dominated by giant sand dunes.
* The Jungle Marathon requires runners to navigate a swamp-infested 125-mile route through Brazil's Amazon jungle over six days.