Running is the most democratic sport in the world and it has never been more popular. More than half-a-million runners finished a marathon in the US alone last year - compared to just over 100,000 20 years ago.
Ultra-marathons in extreme conditions are undergoing huge growth across the globe. But is the age-old sport even good for you?
As the billion dollar running industry races forward, Andy Richardson follows the growth and science of pounding the pavement over time, while preparing to run 42km for the first time - at the Antarctic Ice Marathon.
Tweet your running experiences to #WhyWeRun
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