Hi
If your dress size has stayed the same then I reckon you've lost some fat and gained some muscle. How do you feel about your size (size, not weight)? If you're happy as you are then don't make any dramatic changes. Just stick to a healthy diet with plenty of fruit and veg, some high quality protein and enough carbs to fuel your running.
If you do want to change your size then you need to reduce the calories you're eating while maintaining a balanced diet. Try keeping a food diary for a week to work out how many calories you have now. Then reduce that by cutting out any junk or reducing your portion size. Don't try to reduce it by too much as you won't have energy to run.
I should add that the BMI figures (which I guess you've used to determine if you're a healthy weight) aren't designed for people doing lots of sport so as you run more and build muscle you can expect to be in the upper levels of the range. I'm very slim but still manage to be well into the upper half of my "normal" range.
