Need advice in preparing for Maryland Swim for Life

Hi, I'm signed up for the maryland swim for life on July 13. There is a 1, 2, 3, 4 & 5 mile option and I've chosen the 5. I've read as many of the previous posts here as possible on training. But, I would still like to ensure I'm on the right track and would appreciate some advice. I can always change which distance I'm doing if my training doesn't go well. My Background: I'm a triathlete but I'm awful at swimming. I'm doing this swim to force myself to get better at swimming somehow. I've done some 1.2 mile swims in half irons and pure open water events. I can currently swim 2 miles in the pool without stopping. Recently I've been practicing the Total Immersion method in an attempt to become more efficient. I swim 4 days per week: 3x1500yd + 1x3000yd. Goal: Just finish the 5 mile swim under my own power. Questions: 1. How long should my "long swim" be in order to complete this event? How much should my weekly yardage be? Keep in mind that I've just learned the Total Immersion method and I'm still not that great at it. So, i've been trying to spend a few days a week just focusing on form. 2. Would it be advisable to wear a wet suit? I have a 5/3mm full wetsuit. Water temps are projected to be mid 70s. The wetsuit would give me an advantage and is permissible if the temp doesn't rise above 78F. But, in the mid 70s, would I overheat? 3. Does anyone have any other thoughts on training? Thanks! :-) -Jonathan