I'm having a similar problem, re: my metabolism. Not that I was ever quite so active as you, but as a teenager I did horseriding and dancing, and even at uni I walked everywhere (lots of hills where I went to uni) and did ballet once a week and stretch class once a week too. Now I'm back home and my job involves sitting a lot and I'm the heaviest I've ever been and I hate it.

So far I've been trying to get out on my bike, since that's the most accessible form of exercise for me (along with walking) and obviously trying to not eat endless rubbish - my success varies at that, but the weight won't seem to go. I finally understand the correlation between feeling crap about your shape and eating stuff that will just make your shape WORSE but you do it anyway because you feel crap :/