"Very few people have ever finished New York's 5,000km, eight-week-long Self-Transcendence Race, which really pushes runners to the limit.
"If a jog around the block sounds like sufficient exercise to you, spare a thought for a group of runners in New York who, this June, will lap the same block in Queens more than 100 times a day for eight weeks, in an attempt to clock up 3,100 miles (4,989km).
"The Self-Transcendence 3,100 Mile Race - founded in 1997 by spiritual leader Sri Chinmoy, who believed sport could enhance spirituality - is the world's longest foot race and undoubtably one of the most gruelling. Taking place entirely along concrete pavement and with limited views (a high school, a playground and baseball field), the race takes place from 6am till midnight every day, and runners cover at least 96km a day for 52 days.
"The have a mindset where they don't think about the distance. They just think, 'How many laps do I have to do today?' and break it down that way,' says associate race director Sahishnu Szczesiul. 'The runners don't think of it as a mental challenge, rather as an oportunity to do something for their spiritual lives."
for more on the 2017 Sri Chinmoy Self-Transcendence 3,100-Mile Race