You've got to be pretty crazy to decide you want to run 26.2 miles. Like, that's hours of running, non-stop, with blisters on your feet and legs that must feel like lead.
If you're one of those people who wonder what on earth possesses runners to do the London Marathon, you'll be even more flabbergasted as to why people sign themselves up for these seriously tough-looking challenges.
However, they do take place in some beautiful places... so who knows, maybe you'll end up feeling crazy inspired (maybe).
Patagonian Expedition Race
Teams of four run through lands that may have never been seen before by the human eye. It's in southern Chilean Patagonia, and everyone has to trek, climb, kayak, mountain bike - yep, and even do rope work. Meanwhile, runners can go hundreds of kilometers without seeing anyone but their team members.
The route details are only revealed twenty-fours before it's commencement - but organisers say in 10 races they've covered over 6,500 km.
EcoTrail de Paris
This ultra marathon is a gruelling 50 miles through beautiful hilly trails outside of Paris. Thousands of competitors make their way from the famous Chateau de Versailles, along the Grand Canal, through the Domaniale forest of Meudon and into the city centre, along the Seine into the heart of Paris.
But the finish is perhaps the most exciting part - a climb up 328 steps on to the first floor of the Eiffel Tower late at night to see Paris lit up in all it's glory.
On-course aid stations offer things like cheese and baguettes (you are in France of course).
The Endurancelife Classic Quarter
This 44-mile Ultra Marathon is along the south coast of Cornwall.
The idea is to run non-stop from the southern most point of England (Lizard Point), to the western most tip (Land's End).
Steep climbs and descents challenge runners, as does the particularly unpredictable Cornish weather. Competitors can do it solo, or in relay teams of two or four.
Marathon des Sables
This one, under the Moroccan sun in the Sahara desert, is ranked by the Discovery Channel as the "toughest race on earth".
Rules stipulate runners must carry everything needed to survive - equipment and food - on their back. Except from water that is - and they are also placed in a tent to sleep at night.
Temperatures can reach more than 50C while the equivalent of five- and-a-half marathons is run in five or six days. That's a total distance of some 155 miles.
Jungle Ultra Marathon
The Jungle Ultra is 230km through mountains in Peru, down through the Cloud Forest to the Amazon Jungle below. What's pretty awesome is along the way, throughout five stages, runners may spot wildlife and local tribes.
With humidity levels reaching near 100%, they have to be self-sufficient in this race as well - including carrying their own hammock - as they head to base camps.
Runners cross the Arctic Circle in this 350-mile race. Sleds are used to drag gear along through the rugged terrain - and of course temperatures are way below zero.
This one has a time limit - the course must be finished in 191 hours (eight days). Which is perhaps why only 11 people have finished the event over the last seven years.
The Brutal Extreme Triathlons
Held in Wales, the swim is in chilly Lake Padarn, the bike course goes through stunning scenery (and hills), and the run goes up and down Mount Snowdon.
Apparently voted as "The World's Toughest Triathlon", there are half, full and double distances.
Yeah, you know what? We're quite happy with a 5k parkrun on a Saturday morning, thanks.