WRC lady opining here:
I vote for saying something to her, preferably sooner than later, because it's possible that (a) she & Boyfriend will get engaged (and she possibly might choose differently if she knows you're into her - and you *cannot* tell her after she's engaged, just too late) and/or (b) one of you will move away after med school (and it's much easier to figure out when you live in the same place).
75% chance you're going to lose her as a friend/running partner if above doesn't go well. So, be prepared for that.
You could try something like this:
"Hey, I know you have a boyfriend, and I respect that, and I'm not going to mess with that. But if that job ever opens up, I'd be be interested in learning more about it and possibly applying. I like you and have fun spending time with you."
And then just go about your business.
Good luck, man. (Been there, done that.)