We've known each other...,

[Silverobama]

I told my cousin I'm going to marry soon and he was a bit surprised because I didn't say that I had a girlfriend. I said the woman wasn't someone who I recently met but someone who I had known for many years.

Is my sentence natural to summarize this idea?

We've known each other for so many years before we fell into a relationship.

 

[Glizdka]

NOT A TEACHER
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[...]someone who I had known for many years.

We've had known each other for so many years before we fell into a relationship.

You should use the past tense in this context. I would get rid of the so.

 

[emsr2d2]

I told my cousin I'm going to marry* soon and he was a bit surprised because I didn't say hadn't [even] told him that I had a girlfriend. I said the woman wasn't someone who I had recently met but someone who I had known for many years.

Is my sentence natural to summarize this idea?

We've We had/We'd known each other for so many years before we fell into a relationship.

These days, in BrE at least, we'd say "I'm going to get married soon" or "I'm getting married soon". We rarely use "to marry" unless it's the last two words of a sentence or it's followed by something that describes the person we're going to marry. For example "I'm going to marry my best friend" or "I'm going to marry Sarah".

With regard to the underlined part, it sounds very old-fashioned. I'd say "before we got together", "before we became a couple/an item" or "before we fell in love".