Forget the scales! Do skin fold tests if you want to know how your muscle-to-fat ratio is changing. Lots of research using techniques that indicate body fat more directly (your BMI only is only a 200 year old estimate of body fat composition, not a measurement, it's only still used by doctors because it's cheap) show just how inaccurate it is. A recent study using actual measurements of body fat found the BMI is wrong for half of women and 1 in 4 men. It's inaccurate in both directions: it indicates too much fat in some people who don't have too much (more common in men and younger people) and healthy fat in many people who do have too much (called 'normal weight obesity').
I have a low BMI but I'm all flabby and wobbly, I know that if I got a proper body fat test it'd show I still have too much. Certain parts of your body should wobble, and certain parts shouldn't. We all know which these are. Since I've started running and doing hard resistance training, my arm muscles are already firmer after a few weeks and I feel I can lift, push and open things more easily. If you're just running a lot, then eating a lot of protein might not be enough to preserve your muscle mass, you should really lift weights etc. as well. The last thing you want is to lose a load of muscle, because that muscle loss is the reason that people's base metabolic rate slows down and they gain all the fat back when they try to transition to the maintenance period. Less muscle = lower base metabolic rate, weakened bones, less attractive body. It's not worth the lower BMI, trust me. 
Edited: 29/08/2012 at 14:10