Firstly, I am by no means an expert; having only taken up running in October 2012 but here's my 2p worth.
If you go to my.asics.com and put in your information, it will give you a training plan and a predicted finishing time. You can also download the app and track your runs with that.
It is very rough and ready but will adapt with you i.e. work you harder if you're over performing, and is as good a place as any to start..
Personally, I think you'll be hard pushed to break 2 hours this time as your average pace over 13.1 miles will need to be 11 seconds per mile faster then you're managing 2 miles at present; however, I am more than happy to be proved wrong.
I would recommend aiming for a 2:05 - 2:10 this time and use the base you build to push on for a sub 2 hour around June next year. You'll be less likely to injure yourself and also have 2 PB's within the space of 3 months (assuming this is your first half).
Regarding waterproofs and iPhones, when I used to listen to music on the run, it was over the sleeve; however, as Mat says above, running without music is much better and I haven't run with music for over 6 months now and probably never will again. You're able to hear yourself which really helps control your effort as well as hear what's going on around you. It also means if you enter a race which doesn't allow headphones, you've already trained for that.
I also wouldn't bother with waterproofs. I find that despite the jacket saying it's breathable, the sweat doesn't really escape and you end up being wet anyway. If it is raining and it's cold and I'm doing a long slow run I might put on a light wind stopper, just to take the chill off, but wont bother with it if doing speed work.
I hope the training goes well. Let us know how you get on and hopefully you will prove me wrong about the sub 2 hour.
Looking at the course profile on the great run website and there are some "bumps" between 3 and 4 miles and 5 and 6, as well as from about 10.5 to 12. I did it in 2013 and the last hill is a killer, but the first 2 weren't too bad and that was undertrained, having done very little training since VLM in April. Hopefully, this year I might make it up it without having to walk.
Surely if you let a pacer catch up with you, your chip time is going to be slower than theirs?
I appreciate that if your goal is to simply get round, this wont be an issue. However, if you start in the 4:30's pen and let the pacer for 5:00 catch you, your time will be slower based on the fact you crossed the start line earlier.