long distance swimming

I have a goal of being able to do a 7 to 8 mile open water swim. What kind of training program will get me there? What type of workouts leading up to the attempt? Should I do the full distance at least once prior so I know I can do it, or if I do 5 miles comfortably, then I know I can do the 7? I was going to try it this year, but I don't think I am ready, so I will attempt it one year from now. Also, what type of nutrients and fluids should I have my escort bring? One final question, what is the optimum water temperature for this attempt? I will be using a wetsuit. Is there any precautions regarding jelly fish? Any one with long distances have experiences to share?