Any fitness instructor will confirm that an easy way to add some intensity to your run is to up the altitude. The effect of gravity means you will be breathing thin air, which puts more pressure on your heaving lungs.

Tempted to push yourself to the next level? Consider these five extreme mountain marathons.

Ultra-Trail Du Mont-Blanc 

The Ultra-Trail Du Mont Blanc (UTMB) is an alpine marathon covering Mont Blanc, the highest mountain in Western Europe and the Alps, in one grueling circuit.  As a bonus, runners get to pass through three countries — Italy, France and Switzerland.

The Guardian newspaper asks if the Ultratrail, which runs between August 22 and September 2, is the world’s toughest race. There’s no shortage of contenders for that title, but maybe. The Ultra-Trail pushes runners to cover 103 miles and nine peaks. En route, Guardian reporter Duncan Craig faced sleep deprivation, altitude sickness and cartoon hallucinations to reach the finish. Over 1,000 rival runners dropped out.

Besides the length of the race, entrants in the Ultra-Trail face an elevation gain of some 9,400 m. While elite runners close the loop in just over 20 hours, most runners take up to 45 hours to cross the finish. Underlining the masochism side, there is no prize money. Brutal.

Lowe Alpine Mountain Marathon

Held in June, Scotland’s Lowe Alpine Mountain Marathon is a two-day mountain competition for pairs, featuring an overnight camping experience. The organizers target the event at “connoisseurs.” and highlight the scenic angle. Think deer, golden eagle and game fish: “just don’t let the trout bite your toes.” Underlining the event’s cultural bent, there is even a photo competition. But don’t get too comfortable.

“You will be tackling some very challenging mountain terrain and the daily distances will reflect this. If you are not both competent and confident when moving across steep, rough mountain terrain, you will inevitably be spending longer on the hills than you anticipated,” the organizers warn. They add that you should be ready “for the worst possible conditions” because the competition area is extremely isolated. The hills are exposed to heavy weather. What’s more, once you get going, “you are very much on your own”. So if you want to compete, get in some hardcore practice on mountain terrain. Hone your fine map reading and compass smarts in advance.

Tenzing Hillary Everest Marathon

It would be hard to imagine a more daunting marathon than one that puts you up against Everest. Pitting yourself against the “roof of the world” is a big day out by any standards, especially because despite the mountain’s swelling popularity, it retains the nasty habit of claiming lives. Held in May, The Tenzing Hillary Everest Marathon is named after two of the most famous climbers to tackle it: Sir Edmund Hillary and his sherpa Tenzing Norgay. The Tenzing Hillary Everest Marathon claims to be the world’s most adventurous trail run. During the slog up to Everest Base Camp, runners go through Everest National Park, crossing one of the most famous and popular stops en route to Everest, the Sherpa village of Namche Bazaar. They pass by some dazzling Buddhist monasteries. Then, from the summit of Kalapatthar (5545m/18200ft) they can enjoy the breathtaking views across the Himalayas. If you’re interested in this race, just be sure to familiarize yourself with the tricky tipping regulations documented on the website. And be ready to test your limits

Everest Marathon

Another Everest marathon? Yes indeed. Mount Everest exerts such monstrous allure that it serves as the stage for more than one mighty marathon. Another, simply called the Everest Marathon, is hailed in the Guinness Book of Records as the world’s highest. The start line lies at Gorak Shep 5184m (17,000 feet), near Everest Base Camp in Nepal.   The end is at the Sherpa village called Namche Bazaar at 3446m (11,300 feet). The course extends 42 km (26.2 miles) over rugged mountain trails. So that everyone gets used to the height and avoids hallucinations, runners convene before the race for some sightseeing and trekking around Nepal before embarking on what the organizers claim is one of the world’s “most grueling races”. And one of the most spectacular, they reckon. Be ready for ice, snow and “considerable exposure” to the elements. You need recent experience in related outdoor endurance events and must be accustomed to rough terrain – road marathon experience won’t cut it. The marathon is next scheduled for November 2013. Proceeds go to charity.

Saunders Lakeland Mountain Marathon

Rugged, rainy Britain has a fair few mountains squeezed onto it. One of its biggest mountain marathon events – Saunders Lakeland Mountain Marathon (SLMM) — happens in lovely Lakeland: the English Lake District. The epic two-day event usually unfolds in early July, sponsored by light-weight tent manufacturer Robert Saunders. The SLMM has been held yearly since 1978, except 2001, when the Lakeland fells locked down because of the Foot and Mouth crisis. SLMM is less “hair-shirt” than rival, even older marathon adventure race the OMM. The reasons: the weather is less mean, and the course fractionally shorter. Better yet, the overnight camp is usually a stroll from a pub. But you never know how events will unfurl. Sometimes contestants take a real hammering from the weather – remember that you are in remote England, deep in the northern hemisphere. The marathon is tailored to suit anyone from a soloist to a team player. Because everyone loves the Lakes, which are famous for their hypnotic beauty, SLMM courses take you across the grain of the country, off the footpaths.