So you're all in the "D2D is full of shit" camp?! I disagree. I think she'll do it.
Firstly she's been so definite about this that she knows she'll have zero credibility if she fails. She isn't going to risk becoming a complete laughing stock, is she?
Secondly, for anybody who has run 70 miles a week for a few years, a sub 20 5k isn't a fast time anyway.
So: 19:55
Good luck!