Preparation for St Croix Coral Reef Swim

I'm looking for suggestions on what kinds of distance training I need to work up to in preparation for St Croix 5 mi open water swim this Fall. I swim (25 m pool) 1-1.5 mi/day, 4 days/week, and am now doing at least one 1.5 - 2 mi continuous fr swim per week. What should my daily/weekly mileage goal be to adequately prepare me physically and mentally for the upcoming swim? Thanks for your suggestions!