re: I lied to a girl I'm trying to impress, told her I can run a 5k in under 29 minutes (DONE)

I think the best course of action here is to have a friend break your leg with a baseball bat. Make up a story about something heroic like saving a toddler from a pit bull.

Then show up to the race to cheer her on. She can't catch you in your lie, you score points for showing up anyway, and you'll get all sorts of sympathy points for having your leg in a cast.