About six years ago, I went on a strict 1,200-calorie-a-day, medically-managed diet, and after I shed maybe fifteen pounds, I joined a martial arts studio and started learning aikido.
I went from 225 to 169. It took about six months.
I've put some of the weight back on, but more of it is muscle than before.
The thing about taking up a martial art is that it's very different from other exercise programs. Using stationary bikes and other gym hardware get boring, fast! There's stuff like spinning classes, aerobics classes, etc., but that's one instructor standing in front of a room full of people, like a college lecture - also boring.
When you're learning a martial art, you're actually LEARNING something, a new skill. There's an instructor, but you're also working one-on-one with the other members of your school. There's a more social aspect, it engages your brain more and it can do wonders for your self-confidence. I recommend it to anyone looking to make a positive change in their lives. Aikido in particular, because it's strongly grounded in Zen Buddhist principles - you actually treat your opponent with care and love as opposed to seeking to inflict maximum damage.