Wish I had seen this thread sooner, as now it's 4 weeks into your training. At the time it was definitely possible. The 11:30 is hardly a limiting factor, and although I realize that not everyone is a swimmer, all an 11:30 limits you to is that your stroke is moderately efficient and that you continuously swim. So it's really a matter of getting through the 500 without stopping.
Since the OP probably won't ever read this thread again, I'll post a few tips for anyone else who might read it.
The biggest hurdle for people to overcome when beginning to swim is the idea of putting their face back into the water (and thus cutting off the oxygen supply) while you're still tired and have an elevated heart rate is not a comfortable feeling. Once you get past the idea that you're going to drown (which you're not), your training will be much easier. You NEED to continue to swim without training.
You should be familiar with these ideas as a runner, but here's the 3 major ways of training:
Interval training-Swim XYZ yards on ABC time-
http://swimcoachtools.com/i.aspx-"A training technique where swimmers repeatedly swim a certain distance in a certain amount of time (known as the Interval. For example, if a swimmer was doing 10x100’s on the 1:30, the swimmer would start the 2nd 100 1:30 after they start the first, the 3rd 1:30 after they start the second, and so on. The goal of interval training is to encourage hard swimming, as faster swimming results in more rest."
Rest Intervals- You swim 50/100/200 yards, then rest for 20 seconds, then swim another 50/100/200 yards, then rest 20 seconds. Keep your rest interval consistent, and don't make it so long that your heart rate has dropped and you're fully rested. Make it just long enough that you can get through the next distance with good form.
And once you feel more comfortable in the water-
Fartlek swimming- You pick a long distance (400, 500, etc.) and basically swim continuously. Swim faster when you feel rested, swim slower when you feel tired, and just keep swimming the whole way. Switch strokes if you need to (if your goal time is 11:30, you could probably get away with some backstroke or breaststroke for rest), but just keep swimming.
Hope this helps!