This weekend it's the annual Man v Horse Marathon across hill and dale in Wales. But is it possible for runners to outpace horses?
Originally staged to settle a bar-room debate as to which is faster over a long distance, the Man v Horse Marathon in Llanwrtyd Wells is now in its 30th year.
On Saturday, up to 50 horses and their riders and hundreds of runners are expected on the starting line, all hoping to win the 1,000 guinea (£1,050) first prize.
Although the first couple of races saw easy victories for the horse, the course was eventually altered make it a more even match - and the humans caught up.
Runner's World magazine editor Andy Dixon says the switch in terrain is crucial. "If the ground is rough going, you'd expect a human to cover it at a more or less consistent pace. But the horse has to slow down on a steep hill. It's a bit like the tortoise and hare analogy - the human, being slow but steady, is still able to win the race."
The event - at 22 miles long, technically not a marathon - starts in the town centre and is run over hilly farm tracks, footpaths, forestry roads and open moorland on the edge of the Brecon Beacons in Wales.
The margins by which horses outpace their human rivals have dropped from about 30 minutes to a matter of seconds, and in 2004 Huw Lobb - who ran marathons for Team GB at the time - became the first of only two men to win the race.
His winning time of 2:05:19 beat his closest equine rival by 2 minutes and 17 seconds.
"As an experienced fell-runner I was clambering up steep banks, jumping off ledges and throwing myself down steep hills in a way no horse could ever hope to do," says Mr Lobb. "So while a horse is definitely faster over the flat, I'd say only about 20% of the course I ran was on flat ground."
Mr Dixon points out another human advantage - man's evolutionary legacy.
"Because our ancestors were hunter gatherers, we are genetically primed to cover long distances and outlast our prey. Over millions of years we have developed a naturally efficient running form, so while there may be faster animals, nothing else touches us for endurance."
Another factor tipping the balance toward the human competitor is that horses overheat more quickly.
This is why race officials insist on a 10-15 minute "vet break", says organiser Lindsay Ketteringham, to make sure horses' body temperatures stay within safe levels.
"Weather plays a very important role - both times humans have won the race it has been a hot and sunny day."
Horse expert Dr David Marlin explains the science behind equine overheating. Whereas an 80kg human has two square metres of surface area to shed internal body heat, a 500kg horse has five square metres - so each square metre of a horse's surface area needs to lose about two and a half times more heat.
While horses sweat more than humans, their internal body chemistry means each kg of muscle produces three times as much heat as the equivalent of human muscle.
Not only is the horse carrying extra weight in the form of a rider, adds Dr Marlin, who advises the UK Olympic equestrian team, the animal may not have like-for-like fitness with its human rivals.
"Have these horses been trained to climb hills and traverse rough terrain? If not, it's likely you are matching a bunch of supremely fit, specialist long-distance runners against ordinary horses. The result is bound to be closer than you would at first think."