Well done Clare on your first race and a sub-60 minutes too!
I echo all of the above and it is definately important to see what food you are eating (hidden fats, too much carb etc etc) but you also, quite peversely, need to make sure you are eating enough. I realise your training isn't going to need you eating 3000 calories a day but if you are eating too little your body will try and hold on to all the food you give it and will not want to give up the fat because when you are exercising it panics and thinks there are hard times up ahead so wants to hold onto fat for these "lean" times (some primeval thing i think).
You also need to keep an eye on when you eat to make sure your body is getting what it needs. Protein to help repair, carbs to prepare etc. I am no nutritionist but I take an interest so please take what I say with this in mind and check anything.
Where are you at the moment fitness/running wise (taking into account your operation what were you like before the op)? That will probably depend on where you need to start from. Also how long before your fitness test do you expect to be able to exercise and then run?
12:30 for a 1.5 mile run is about 8:19 pace and 10:30 would be a 7 min/mile pace.