I use Greenbelt station daily (albeit as a bus rider, but still).
The long-term parking spaces are located on the extreme north side of the lot, just west of the entrance from the Metro access drive from Cherrywood Lane. Because they're so far from the station, they're not very much in demand by commuters. The only time I think you'd have trouble finding one of those spaces is on a day with a huge event downtown. Like the 2016 Inauguration or the Fourth of July.
To describe where they are, if you're driving into the station from Cherrywood Lane, there's only one entrance to the parking lot. Turn left into the parking lot. After you go through the gate, make the first right, and then an immediate right so that you're headed back toward the access road. The spaces directly ahead of you that are alongside the access road are the long-term spaces. They are only marked by signs, not paint or anything.