I think this is where you get the impression:What I wanted to say is a phrase with 没 doesn´t use 了, as 没 indicates past, 了 is not necessary
You have to move 了 to the end of the sentence.
If you said 我|好久|不去|长城了，I would think you deliberately don't go there for a long time.
If you said 我|好久没|去|长城了，I would think you just haven't been there for a long time.
("|" is for better understanding.)
The 了 has nothing to do with tense here. It's a 语气词. The meaning of the sentence wouldn't change if you omitted it.