Length: I think it was a longish one-shot (8,000 words or less) but I'm not 100% sure
About: from what I can remember it starts off as an 'on the run/getting out of New York because something happened' trope where Steve and Darcy are in a car going to a secure location (where the other Avengers are waiting), things are kind of strained between them, there's some tension and Steve isn't overly nice to Darcy. Then when they get near to where they're heading the car breaks down/runs out off gas or they can't get there by car and they have to walk to get to where they're meeting the others (I thinks it's Clint's place?), they hike through the woods but Darcy can't keep up with Cap, they have a disagreement, then it starts to rain so Darcy heads for shelter and Steve goes off in a huff but comes back and emotions overflow/sexy times happens. In the end when they meet up with the others there's something about Thor making the storm happen so they would get together and Steve/Darcy end up rooming together...and that's all I can remember.
Hopefully someone will know this fic :)
(Edit 08/04/15 - found in comments)