For most runners, completing a 5K, 10K or even a marathon represents the defining moment in their sporting calendar. However, others drive themselves even further. In the pursuit of pushing their limits, a small group of athletes hunt down the extremes.
Whether those are extremes of heat, cold, altitude, or simply their own psychological barriers, there are an increasing number of races and events that challenge athletes to push themselves further and faster than ever before.
As such, creating a list of the world's toughest races is, in itself, challenging. This list of races isn't laid out in order of 'toughness' - they all have their own uniquely challenging attributes. However, each of these legendary events are renowned for pushing athletes to their mental and physical limits.
Self-Transcendence 3100 Mile Race
Distance of challenge: 3,100 miles / 4,989km
There's running in loops, and then there is this. At 3,100 miles / 4,989 km the Self-Transcendence 3100 Mile race is the longest certified footrace in the world. It's also, perhaps, the simplest ultra marathon on this list. Runners complete loops of a single block in Jamaica, New York. The goal is to run a minimum of 60 miles / 90 km per day, and the direction of travel - whether the runner turns left or right on the corners of this block - changes from day to day. That's it. At the end of the race, runners arrive back in the very place that they began, so completing their 'journey' of self-transcendence.
The very nature of this race means that, while there are 'winners' and course records, the real goal is for athletes to tackle the obvious physical and psychological pressures that come with completing a race like this. Kareenika Janakova holds the women's record for the race - 48 days, 14:24:10, while Pekka Aalto holds the men's record - 40 days, 09:06:21.
6633 Arctic Ultra
Distance of race: 380 miles / 610 km
A race against the clock and - at times - a battle for survival, the 6633 Arctic Ultra tests competitors on every level possible. As the route winds North from Eagle Plains, Yukon, temperatures regularly plunge to -40C / -40F, while Arctic winds whip across the tundra. Almost entirely self-supported, competitors cross the Arctic Circle (the race takes its name from the latitude and minutes of this legendary 'barrier') en route to the finish line.
There are two event options - the 120 mile / 193 km route that finishes in Fort McPherson, or the full 380 mile / 610 km foot race that ends in Tuktoyaktuk, which sits on the edge of the Arctic Ocean in Canada’s Northwest Territories. Completing either is a supreme feat of physical and mental endurance.
Mimi Anderson holds the current course record at 143 hours, 25 minutes.
Distance of race: 152 miles / 246 km
Every running race has a history, but few have a history as storied as the Spartathlon. During the Battle of Marathon in 490BC, a Greek messenger by the name of Pheidippides was sent to Sparta to request reinforcements for the beleaguered Greek forces. According to the accounts of the historian Herodotus, Pheidippides had completed the 152-mile (246km) trip along goat tracks and over mountain tops by the next day. A legend was born.
To this day, the route - and indeed many of the tracks - of the Spartathlon have not changed. Every year, more than 300 athletes attempt to complete the course by passing through 75 checkpoints - each with a strict cut off time. As such, the Spartathlon is as much a race against the clock as it is against competitors.
ITI 1000 (Iditarod Trail Invitational)
Distance of challenge: 1,000 miles / 1,609km
Alaska in the winter is a truly unwelcoming, unforgiving environment. Even moreso when you're running through the wilderness. The ITI (Iditarod Trail Invitational) 1000 pits some of the world's toughest ultra runners against some of the world's most brutal conditions.
A direct descendent of the legendary Iditarod dog sled race, the ITI 1000 follows an unmarked route through Alaska from Anchorage up to Nome on Norton Sound. There are a series of checkpoints along the way, and competitors can pre-supply themselves by sending consumables to villages along the way, but make no mistake: this is a self-sufficient race.
What's more, the majority of it takes place at night. Through the winter, this part of the world receives precious little sunlight, meaning competitors race through the Arctic twilight, tackling mountain passes and frozen rivers. Along the way temperatures can easily drop to -46C / -51F and winds of anything up to 50mph / 80kmh have been recorded (wind chill reduces those temperatures to -90C / -130F).
Tim Hewitt holds current men's course record is 19 days, 9 hours and 38 mins (2016), while Loreen Hewitt holds the women's record - 26 days, 6 hours and 59 minutes (2014).
Distance of race: 135miles / 217 km
Stark, dry, hot. California's Death Valley is not a place that many people choose to go in the middle of July. The mercury regularly hits 120F / 49C, and the flat salt plains suck any moisture out the air. But every year, a small group of runners set out to tackle a truly grueling race - the Badwater Ultramarathon. The 135 mile / 217 km route begins at 85 meters / 280 feet below sea level, and finishes at the Whitney Portal (2,548 m / 13,000 ft). In the early days of the race competitors used to have to summit Mount Whitney (14,505 ft / 4,421 m) but local authorities put a stop to that as the race grew in popularity.
Needless to say, some intrepid competitors still make a summit dash, while others choose to run back to Death Valley and complete the 'Double Badwater'. Indeed, a select few have gone further, with the 'Triple', while legendary ultra runners Marshall Ulrich and Lisa Smith-Batchen have completed the Badwater 'Quad'.
The course record for the 135 mile / 217 km course is 21:33:01 set by Yoshihiko Ishikawa (2019). The women's record was set by Ashley Paulson in 2023 - 21:44:35.
Tor des Glaciers
Distance of challenge: 279 miles / 450 km
The Tor des Geants has long been renowned in ultra running circles as being one of the toughest trail races on the planet. Not content with that reputation, the organizers decided to up the ante to commemorate the tenth anniversary of the race with the Tor des Glaciers. An epic endurance test for just 200 experienced mountain runners every year, the 279 mile / 450 km trail race boasts a staggering 105,000 feet / 32,000m of elevation gain.
Taking place around the Aosta Valley in the heart of the Italian Alps, the race crosses relatively untrod ridges and mountain passes that regularly exceed 6,560ft / 2,000m above sea level. This challenge is even greater given the semi-self-sufficient nature of the race. Throughout the course athletes pass by refreshment points serving food and drink. What's more, at certain pre-defined spots they are able to receive assistance from their team. However, away from these points athletes are on their own. As such, they have to ensure that they have everything they need to survive in the mountains.
Sebastian Raichon holds the men's record - 123:57:18 (2022), while Stephanie Case set the women's record in 155:06:00.
The Barkley Marathons
Distance of challenge: 100 miles / 160km
Every single thing about The Barkley Marathons is hard. Simply getting a spot on the start line is tough - there is no website or 'normal' entry form. Instead, prospective athletes have to write an essay about why they should be allowed to participate. If they are successful, they pay the $1.60 entry fee, and bring a car license plate from their home country to the starting line somewhere in Tennessee's Frozen Head State Park.
The race begins with a trumpet call at an undefined time on a secret date. Participants then tackle five loops of a 20 mile / 32 km course - but given the difficulty of the terrain athletes usually end up covering closer to 26 miles / 42 km. Each loop contains around 60,000ft / 18,288m of elevation gain, and the same again in descent. There is no formal race tracking, but to ensure athletes don't cheat, books are placed at checkpoints along the route, and every athlete has to tear out a page to prove that they have passed that checkpoint.
Fewer than 2% of first-time participants have been able to complete the course, and only 15 runners have managed to finish the race since 1995.