For some people, one marathon is a lifetime's dream.
Others get the bug and go back for more, while the more hardy - and some would say foolish - push on into ultra races or towards the 100 Marathon Club.
Steve Edwards, the self-styled Man Inside The Machine, takes all that to another level.
He has already run 903 marathons, and is aiming to reach the 1,000 mark.
As if that isn't impressive enough, Steve only counts official races - no virtual runs for him, doesn't count marathons he has to walk in and is aiming to get to his goal with an average time of less than three hours and 30 minutes per marathon.
Unsurprisingly, he's set a host of world records along the way, including becoming the youngest person to reach 100 marathons and the most marathons run in a year.
I spoke to him about how and why he started out on such an enormous task, and how he manages to stay fit when running so much...
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