Granty gives sound advice..I'd add that if you can you join a local running club to help keep you going and for support.
Also get your running shoes from a proper running shop...not from the likes of JJB.
Assuming all goes well with your training try to do at least two 12 mile runs before the race.
It's a good idea too to enter some shorter races before hand to get used to race running and all that goes with it...ie pre-race prep.
What made you enter the Plymouth 1/2M?
All the best
So am I and it will be my first time I'm also following Geoff Galloways plan but you don't have to start at the beginning just start at the point that you've reached on your long Sunday runs - take Steve's advice as well and consider joining a running club there are 3 in Plymouth.
I'm assuming you went on this months Brooks run and have joined the forum in response to Dottys email?
Steve - I assume Jo entered the Plymouth Half as lives in Plymouth - have you ever run it?
Best thing you can do is get down to Frank Elfords on Mayflower Street. Take your old trainers and tell them you are training for the Plymouth Half alternatively if you can get to Launceston - I'd go to Lauceston sports they have a treadmill and do a gait analysis - Frank's don't do this they just look at your shoes.
The Brooks runs are not a race they are a training event held on the first Sunday of every month up at Burrator and as you are training for the Plymouth half you will really benefit from them. They split you up into ability groups so don't worry about not being fit enough - you can find more information here: http://www.city-runs.co.uk/html/plymouth.html#map
I have had a couple of Asics (always last years model - much cheaper !!) and never had any blisters or anything else, I chose those myself and was advised by someone to always get a size bigger that you actually are.
However, as you are starting from scratch it might be best to go to a running shop and get someone to check you out and 'hand' select a pair to suit your running style.
I agree with all of the above, come to Burrator on the first sunday of the month (about 0830/0900) there will be lots of friendly runners of various abilities ready to offer support and advice. Remember, weather is not an excuse!!