Despite what several major-media organisations may have reported in recent months, the first astronaut to run a marathon in space was Sunita Williams. She ran the Boston Marathon in 2007 onboard the International Space Station.
The Telegraph (UK), ITV News (UK) and the Courier Mail (Australia) all seem to think that Tim Peake will be the "first person" to do this during tomorrow's London Marathon.
ITV News incorrectly claims "Tim Peake will become the first ever person to take on a marathon while in space"
Given their publication dates of these stories, I imagine that they read the Principia Mission blog, saw "Tim Peake will become the first man to run a marathon in space", and didn't appreciate that "man" was a very necessary qualifier to the statement. Converting "first man" to "first person" may be due to an unconscious assumption that men set records first. It also suggests that these stories were written by people who didn't have any relevant background knowledge or time for basic fact checking.
US news outlets (including the Washington Post and Fox News) have remembered that Sunita Williams already did a marathon. Hopefully that isn't just because she is American.
Good luck to Tim for the London marathon tomorrow morning. He could break Sunita Williams' current record for completing a marathon in space.