Fitness and weight confusion

I recently decided I’m not fit enough. I used to get plenty of exercise, with a three-mile dog walk twice a week, a weekly couple of hours of dancing, and daily mile-long walks into town and back. Unfortunately the dog is now only around on days I’m not, we no longer have the exhausting dancing practice sessions and the daily walk is getting boring, so only happens a couple of times a week when I force myself (I need a new route). So I’m basically doing nothing aerobic. 

This is all totally pants, and what with research showing aerobic exercise is absolutely vital in keeping the brain functioning at reasonable levels, won’t do. I’ve also been noticing that my stomach sticks out much more than it used to, which is undoubtedly down to too much chocolate, and I really don’t like it. These two were enough to finally spur me into action.

So I decided on a new regime involving cycling and using my parents’ cross-trainer twice a week. This got off to a slow start, but I’m now forcing myself to do it. I spent 30 minutes on the cross-trainer this evening and felt appropriately exhausted, which was the intention. Then I weighed myself.

I’m as light as I’ve been in years. And years. In fact, the BMI calculator reckons I’m underweight (edit) below the recommended minimum. WTF. Are my muscles wasting away? I know fitness and weight are different, but still, that’s just weird. Maybe the scales are broken. I’ll see what happens if I keep exercising – hopefully I’ll get heavier?