I started to read this probably more than two years ago and I never finished it. Oh man this is gonna be hard to get.
I remember that the fic starts telling the reader that Dean goes jogging every morning before breakfast and this one morning, when the story starts, Sam comes with him and I think he was wearing one of Dean's pullovers. It's all very unclear, sorry.
They run down the road and end up in a field, making out. The morning dew was described in detail and that it was still cold and damp for the time of year. I also remember that they didn't live in motel but John had rented a farm house that was pretty isolated.
Sam was definitely a teenager, maybe Dean was, too.
If you can find the fic from this description you are made of win, period.