You can say "he hasn't gone fishing for a long time" instead of "he hasn't gone fishing in a long time". The preposition "for" is often used to indicate "over a period of time". The preposition "in" is often used to express action to occur after a period of time.

No, you can't. In fact, Side By side is the best textbook around. Learn all the little phrases there exactly as they are given. You will learn a lot of very useful stuff there. "I will go to Beijing in two weeks."...is a phrase without any meaning. It doesn't imply anything. Once again, follow Side By Side exactly as it is given and remember all the context there, too.

Well, in the US, we can say "I haven't been fishing IN a long time" or "I haven't been fishing FOR a long time." The meaning of those two sentences are the same. Also, in American English, "I will go to Beijing in two weeks" definitely does have meaning. It's the same as when British people say, "I will go to Beijing in two weeks' time." Both are correct and the difference is just related to US vs UK culture and usage. Anyway, why are you looking at Side by Side, April? It is way below your level! :)

I agree with Randy..." for a long time" sounds more nature...