How to start training for long (to me!) open water swims

I swam in USS programs while growing up through high school and played water polo through college. I have also done several triathlons (sprints and olympic distance). However, I am at a loss as to how to prepare for an upcoming 4.5 mile swim in the Chesapeake Bay despite looking through these forums (there are some great posts here!). I can usually fit in 45-50 minutes per weekday (translating to 2500-3000yds in a 25yd pool) and I'd like to start adding a longer swim at least one day on weekends. But what do I do during this time? I'm used to sets of 100s/200s/400s, but I can't imagine that will cut it anymore. My stroke is finally getting back into form after a two year hiatus from the pool so I'd like to target these workouts for more efficient swimming and more endurance. Any advice for somebody new to long, open water swims would be greatly appreciated. Thanks!