Have just stumbled across this thread and hope you don't mind if I add my tuppence worth...........
Very well done on the weight loss and the improved fitness Andy.
While I agree that your doc should have been a little more encouraging, I would like to point out that it is possible to become annorexic if you are doing too much excercise and not taking in enough calories.
I coach junior middle-distance runners and sadly have seen this on more than one occassion....... parents not realising that due to the amount of activity their kids are doing their diets should be adjusted to give them the correct amount of fuel.
Whilst you were trying to loose the weight then obviously you needed to burn more calories than you were taking on, but if you have reached an optimum weight then you really should just check that your calories are at a maintanance level.