www.theguardian.com/lifeandstyle/2017/jun/16/experience-went-into-labour-after-running-marathon
The comments are great - 50% people saying it was unwise (but not saying why) and 50% women saying good on her, and of course it's fine!
She took it easy (!) - took her over 6 hours.
I'm not built for speed personally, but I've spent hours walking trying to eject both my kids, it's recommended in fact (couldn't have run, and at a very slow walking speed due to ratio of baby to internal capacity making it all a bit tricky so I doubt I did a marathon length, but a fair few miles were walked!).