Nice score on the desk and return. Thank you, neighbors! First, play with the room - don't stress over it. The room dimensions with the 7' walls are a challenge, but if it were my office, I'd try the desk in front of the window, either facing the window (preferred because I love light) or with my back to the window. Glare from outside could be a factor depending on direction the window faces and so could view. Return/credenza doesn't have to go behind desk as is done in typical offices - it can go where needed or useful - maybe an L-shape arrangement, maybe elsewhere in the room. Depends too on how much and for what you use your office. The opposite ends of the room could be a library/sitting/reading area on one side and maybe some sort of combo storage/game table on the other. I'm sure other Houzzers will have lots of ideas and we'll want more pix. Have fun with it.

I would try return along window wall & desk perpendicular as the room is long & narrow - this will create 2 areas - an office area & perhaps a reading area at other end. Also with this placement u will get natural light from window and not have your back to door when sitting at desk - I have attached pic that sort of gives idea. U could add shelves along short wall behind desk for display or storage, u could add bookshelf unit at other end with 2 comfy chairs for reading

Pic for other end of room