Question for Marathon swimmers

I have a question for marathon swimmers... I've been prepping for a 10k swim that I'll be doing this coming Saturday. I live in coastal southern New England. I really began training in late Feb or Early March by doing longer pool swims. Once the open waters warmed up enough for wetsuits, I moved outside in late May/early June. I completed a full 10k+ with my kayaker today and practiced feeding/drinking him. I feel ready. In the future, I'd like to maintain fitness for doing swims of this long, and longer. But my question is...how do you maintain fitness for doing swims of this distance through winter? I just don't know if I can handle six months of long pool swims. Dan