Regarding your target time of 30 minutes for the open water swim, I think you'll find that quite aggressive based on the fact that you're holding 1:30 for individual broken 100s. Also keep in mind there are several other factors to keep in mind such as waves, current, wind, etc. If you're allowed to wear, and are wearing a wetsuit, you'll swim faster due to the added buoyancy. However, even in a calm lake with a wetsuit, I think you'll find 30 minutes a pretty aggressive goal. It's tough to set a goal for the first open water swim in a while. Even if you have swum the same body of water, environmental factors can completely change the race unless it's a smaller lake and consistently calm conditions.
Swimming open water is very different compared to 25 yard pool swimming. When swimming in a 25 yard pool, you're only moving your arms for about 75% of the lap, depending on how long your push offs are. In open water, you don't get that extra push off every 25 yards. It's a little break and a little boost every 25 yards that you're getting in a 25 yd pool that you don't get in open water.
I recommend you find somewhere you can swim a mile, or any distance, in open water a few times before your race. You should be able to find a group of swimmers, without going to far, that get together for open water swims. I would base your target time off your open water practice swimming times.
My opinion on your workouts is that they sound more like short distance workouts than long distance workouts. I recommend you get to the point where you are doing 3 or 4 x 800s after warmup, trying to hold 1:30s, which would be 12 minutes per 800 plus resting time. Don't take more than 30 seconds rest between each 800. Maybe work your way up to 2 or 3 x 1000s, and then to 2 x 1500s, staying as close to 1:30s as possible. I wouldn't do these longer distances every day, but at least once or twice a week, especially as the race approaches. I would also swim at least a mile straight every couple weeks. You would be better served to swim more than a mile, every couple weeks, if you plan to do it in a lap pool since you really should swim around 1.5 miles in a 25 yard pool to try to equate to a mile in open water.
Lastly, since you're just getting back into swimming after a long time out of the water, make sure you're technique is accurate, and be sensitive of shoulder pain. Don't burn out your shoulders. Keep your elbow bent and hand close to your body so you're not taking too much pressure on your shoulder joint.
Good luck!
-Chris
www.theathletevillage.com
