Nick,
the other thing to consider is that if you run (which you do) and you consider yourself to be carrying too much excess fat (which apparently you do - I can't comment), then you will have the double wammy (weight wise) of muscle plus fat combining to give you a higher BMI.
If you continue to train for endurance events then your body fat percentage will probably come down (calories in permitting), eyt your muscle mass will probably also continue to grow slightly, so the reductiion in your bmi may not be proportional to the amount of body fat you lose.
As a tool for the sedate masses bmi is a useful indicator and no more, as an indicator of fitness for purpose in anybody who does regular exercise and wishes to improve competietively its about as useful mas the aforementioned chocolate teapot. IMO
At the rsik of starting a heated debate, I regularly use a set of Tanita scales to monitor bmy body fat and water content. These also make assumptions about body make up, but are more sophisticated than the bmi tables and allow you to define by sex and exercise habits. Since I train 10hrs a week or more in one way or another I qualify for the "athlete" setting - which gives me a good feeling in itself!!