I would invite the girl for coffee. Tell her that she looked like a great runner from her parkrun, and that she is pretty good for a girl. Tell her that you can see potential in her, and that there is a bit of Bekele in her.
Find out her other interests too outside of running. When she tells you her other interests or hobbies, tell her that you bet she is really good at it. Tell her that any guy would be lucky to have her and that you would choose her over a parkrun WR. You should compliment the fukk out of her to make her happy. It doesn't matter if the compliment is not true, she probably won't be clever enough to pick up on it(this is what I learnt from a pick up artist, you need to be an ashole or have the mindset of one sometimes and not a nice guy).
Also you need to be funny. Ask her if her legs are tired because she has been running through your mind all day.