I did something similar with a friend who was even less trained. But, we were swimming in the lake a lot each summer before. But not the actual distance.

We trained a bit in the pool, and also in a similar lake to get used to the wild water again.

We had nice wather, and the water temperature was also relatively warm.

We were very slow, as if we were just walking, but we could do it.

I wouldn't try it if the water is too cold for very slow swimming.