Lovin all the WOMANLY advice here.
First of all, I'm glad you're NOT that guy who goes to the gym to pick up on the gals. I hate that kind of guy.
Does she have a certain treadmill she frequents? If she does you should find yourself on the one next to that one at the gym sometime. When she gets on or off the thing just casually comment on her running and then ask her if she's currently training for anything. This will probably lead into a conversation about how you are training for a marathon...and blah blah. Before you ask her about hooking up to do long runs you might want to ask her what pace she runs at. If it looks like a match throw in the long run possibility. I'd think if shes running TO the gym and then AT the gym, shes probably going faster than 10:30 pace-but its worth a go!
She does have a specific treadmill she frequents...the one that's open. hahaha. (been crazy with the resolutioners...surprisingly not many have dropped off yet...i give them a few more weeks).
I am not sure what her pace is but you do raise a good point. No point in getting smoked by her on a run.
Problem is she is in the gym, on the treadmill, on the elliptical and out of the gym. Gonna be a challenge getting 2 seconds to engage her in conversation.