Pretty interesting profile of Tracy Barlow from Athletics Weekly. Barlow was a sprinterr in HS and was doing the 400h and jumps as recently as 5-6 years ago.
After being inspired by watching the London Marathon in 2010, she got a charity entry to the event in 2011 and broke the modest four-hour barrier with 3:52:59. Then, in 2012, she improved to 3:27:18 (in London) and 3:17:45 (in Amsterdam) in 2012.
The three-hour barrier fell in London in 2013 when she ran 2:59:53. Soon after, she began being coached by Nick Anderson and her PB improved to 2:51:29 at Berlin in 2014, then 2:39:56 in London 2015, 2:38:49 in Toronto 2015, 2:33:20 in London 2016, 2:32:05 in Frankfurt 2016 and now 2:30:42 in London in April.