Reporter Martin George completed the race in six hours, running it for the Swaffham and Litcham Home Hospice.
Tweeting throughout the race, here's his reaction after crossing the finish line:
'This was the hardest physical activity I have done in my life.
'The first half seemed really pleasant and I was even enjoying myself giving high-fives to children and excepting sweets from strangers.
'Because I was Tweeting as I went everybody's messages of support really boosted me.
'The highlight was going over Tower Bridge, just before half way, but then it all got a lot tougher.
'At one stage most of the people I was running with were walking, so it was really hard to motivate yourself to keep going.
'But once I got to the Tower of London the crowds really picked up and they were calling out my name or giving me a wave and it was exactly what I needed to get through the last four miles.
'One thing I didn't expect was how emotional I got at the 23rd mile, I had to try and stop tears welling up, it was weird.
'When I got to the Mall I was half smiling and half crying, it was such a relief to have done it.'
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