Weight gain and exercise

Hi!

Firstly to say that this is a great site, packed full of information.  I have changed my diet over the past 6 months after advice from a personal trainer. During the first couple of months, I lost a couple of kilos, felt good and my weight stabilised nicely (I'm not what you'd call a skinny woman, usually at the top end of the 'normal' BMI weight band). Then Christmas intervened, but I only put on 1-2 kg, which is good for me. Then I started training for a half marathon, and have upped my weekly mileage from around 10 to 20 plus. 

The Xmas weight has not shifted, and in fact I appear to be getting bigger (especially around my thighs) and gaining weight.  I am not particularly athletic in build (more curvy / sturdy).  You may say that muscle weighs more than fat etc, but I actually appear to be slowly gaining inches (not just around my legs).  I'm not consciously eating more, but can only think that this is the case!  Any tips on the exercise or eating front? I don't want to end up fit but fat. My half marathon is at the end of March, so I'll probably revert back to running 3-4 times a week with a lower overall mileage after that. Many thanks!