How to train for a 6-7 mile open water swim

There is an open water swim that’s between 6 and 7 miles from an island offshore to the main land in New England this summer. I would love to do it but the furthest I have ever swam is 2.4 miles in a lake during an Ironman event. I swim in the ocean and I am comfortable with that, I’m just not sure where to begin with training for a distance like that. Thank you!