I think they are purposely misleading with the locations in the show in regards to their real world locations. They don't want to get bogged down with the details and try to explain the actual distances and whatnot. So they are very loose with the names and places so we can't lock down where these places really are.

 

Oceanside is on the Chesapeake Bay, so definitely not Allegheny. There are actually no national forests on or even relatively near the Chesapeake Bay, so basically as @Chr1s said, we are likely dealing with fictional geography. Alexandria itself, while an actual city in Virginia, seems mostly fictionalized for the show. The real Alexandria sits right on the Potomac River, but we've never seen a hint of it, and if that were the case they would use the river [a tributary of the Chesapeake], to travel to Oceanside. It would be both easier and much safer. From hints we've picked up over the years, based on dialogue and directions they travel to get to certain places, it seem that TWD's Alexandria is more centralized in VA.

Rick Grimes also calls the fictional King County, GA, his home.

In the next episode, depending on whether they travel north or south in the bay to get to bloodworth island, we might get a hint of where along the bay Oceanside is.

 

Some random thoughts:

Weren't they heading to Oceanside from Hilltop? If so, going through Waterford may not be out of the question if Hilltop were located in the piedmont area, say where Lovettsville is located.

Prince William Forest is a park owned by the National Park Service and sits about 10 miles (16K) south of Alexandria in the real world. Plenty of people in the area refer to it as a national forest, so I can understand why the characters would refer to it as a national forest.

I believe the writers use names of local places to give the viewers a sense of where the action is taking place but they mix up the locations of the places.