Some run for fitness, some for mental clarity and some for charity.
For some, it's the challenge. For Shona Thomson running began as a challenge and then, somehow, became a little out of control.
"It wasn't even a drunken bet. I just wanted to push myself, to be stimulated and challenged. I am the type of person who needs something to aim for in life and, without a goal, I can get myself into a bit of a negative state.
"Running literally provides direction as well as the euphoria of the 'runner's high'.
"I really enjoy pushing myself, not necessarily at the time, but the resulting sense of a achievement far outweighs any temporary discomfort."
Thomson, who grew up in Craigmaddie, near Glasgow, took her need for a sense of achievement to a whole new level; last year she became the first Scottish woman - and only one of 20 - to run a marathon on every continent.
As one of the highly exclusive Seven Continents Club, which has fewer than 100 members including Ranulph Fiennes, Thomson is now on a mission to speak to schoolchildren and encourage them to push themselves, to learn that extraordinary achievements can belong to ordinary people.
Despite this, Thomson is modest. "I don't think what I've done is terribly special. Lots of people run marathons raising millions for charities and in much quicker times. Many people have overcome significant hardships in life and gone on to accomplish amazing feats."
Thomson began running at school, taking part in cross country, but gave up when she reached university. Eight years ago, having moved from Scotland to work in the City, she started running again, completing 5ks then 10ks and adding a little extra each time.
Eventually running for two hours in a go became easy and the idea of a marathon became plausible, namely the New York Marathon in November 2010.
At that point the American adventure was to be Thomson's first and last race of that distance. Once the box was ticked, the thought of continuing with such a gruelling schedule seemed out of the question, particularly when her health took a downward plunge. A bout of shingles was followed by lower back pain and a shin splint, all of which stopped her running for two months.
On top of that, she was also severely anaemic. Her ill health affected her mood and she began to feel low, directionless.
"I knew I needed a new goal and something endurance-based seemed an obvious starting point. I began reading about the Comrades marathon in South Africa and I knew I had to do it."
The race, 55 miles from Durbin to Pietermaritzburg, is infamous for its torturous climbs through the Valley of a Thousand Hills. "The little fire in my belly was ignited. It sounded brutal but perversely this made me want to do it all the more."
People hoping to take part in Comrades must run a qualifying marathon and so Thomson began researching other races, leading her to the Seven Continents Marathon Club.
To join, runners must complete the Antarctic Ice Marathon, the only marathon run on the interior of the Antarctic, as well as a marathon on all the other six continents.
To Thomson, who is in her 30s, it was exactly the jolt she needed.
At first, she planned to run all six continents in one year but her career in investment banking put her ambitions on a peep and, instead, she plotted to run three marathons in 2012 and three in 2013.
However, injuries forced her to again change her plans. Just 10 days before the London marathon she strained her right thigh and spent the lead up to London resting and treating her leg with ice. Thomson still managed to complete the event, with the Comrades race looming just six weeks later.
By this point, Thomson says she was feeling tired and slow and running to work and at weekends sapped all her strength. But Comrades made the ordeal worthwhile.
"Few other races in the world reflect the courage, endurance, stamina, determination and the human spirit more than the Comrades ultra.
"The rules of the race are brutal. It is run 'gun to gun' with a 12-hour cut off. Anyone who does not complete the course in 12 hours is not allowed to finish, they are removed from the course, no medal, no recognition for the months training.
"It was at the 70km mark that I began to feel I needed to dig deep. I had heard that you run the last 20km of this race with your heart and this was true. I passed bleeding runners, limping runners, runners lying on the edge of the road, grown men crying, people vomiting and lengthy queues outside the physio tents.
"Despite the tiredness, the thought of stopping didn't even enter my mind."
Next came Antarctica, Thomson joined 59 other runners in -40C temperatures and 24 hours of sunlight. It took five days of waiting for the perfect conditions before the race could start, leaving runners alone in some of the harshest conditions on earth.
"Running in Antarctica is an experience that is hard to put into words. Not phased by the pre-race briefing that centred around warnings of frostbite, lost fingers, hypothermia, snow blindness and crevasses, we were piled onto a Soviet war plane, complete with paratrooper ropes, to fly from Punta Arenas in Chile to Union Glacier Camp.
"It was a very isolating experience. There were no crowds to cheer you on. The field spread out quickly so the nearest runner was merely a black dot against an expanse of ice."
Next came Rio for South America in July 2013, Perth for Australia in August 2013 and the inaugural marathon in Da Nang, Vietnam in September 2013, meaning Australia and Asia would be run consecutively.
Rio was "gruesome", despite the glorious scenery the weather was humid and 30C. "I don't think I will ever forget the last two miles of that course. I had well and truly hit the wall."
Perth was a "favourite" race in perfect conditions - low temperatures, drizzle and a cool breeze. In Vietnam - "stifling" - the race began at 5am to avoid the heat. At that time of the morning it was already 27C and, at the start line, the race director advised competitors to run only a half marathon due to the difficult conditions.
Thomson's efforts saw her nominated for the Scottish Adventure Awards earlier this month. Despite being shortlisted for Adventurer of the Year and Inspiring Others, she didn't win her categories but says she was honoured to have been recognised.
Of course, someone who pushes themselves to such extremes is unlikely to give up. Thomson is now training for a marathon in the Geographic North Pole, which she will run in April.
She is also working with Dr Andrew Murray, who promotes health and fitness with the Scottish Government, as well as being involved with the Dame Kelly Holmes Legacy Trust an volunteering as an ambassador for the 5x50 Challenge in which people run, walk, jog or cycle 5k every day for 50 days. Thomson is also looking to mentor others starting out on their first half or full marathons.
"I think the key to everything is not being afraid to fail. No one ever gets anywhere if they sit in their comfort zone.
"It's also important to find a goal you're passionate about and commit to it.
"I have learned to be more patient and flexible. Stubbornness, or I prefer to call it determination and Scottish grit, is also critical. Most of all, the ability to handle pain and keep going. That's a message I want to pass on - especially to schoolchildren. Find something you love in life and go for it. This might have been said a thousand times before but if I can do it, anyone can."