I've lost 25% of my body weight this year (I started on 4 Jan) -- 16 to 12 stone, ~100kg to 75kg. I've run marathons and climbed mountains in the past -- big Himalayan lumps and I've sumitted Denali, for example -- so I know what fit means. Or at least my body does.
Exercise comes naturally after losing weight, It is incredibly hard when you are overweight. At the start of the year I would sweat walking around the supermarket. Now, I run six miles over the local hill for an hour and a half. And it's marvellous. But it isn't easy. It requires single minded, selfish determination.
(Five years later I'm back up to 95ish, but I'm on the way down again. Remembering what it was like is a good motivator.)
I was always a skinny guy most of my life. 4 months ago, I was 5'11", 140lbs, really lean. I recently got serious about weight training and bulking up, lots of high calorie/carbs/fats/proteins, and I'm now up to 177lbs. I'm not exactly fat, but that extra weight is killer. All of my movements are slower, walking up stairs gets me out of breath. I never really understood how much extra weight affects people until now.
I'm not anti-exercise and I've worked out a lot in the past, but I've found that being skinnier makes being active easier and more fun. Moving sucks if you're carrying an extra 50-100 lbs. around. Most skinny people don't really get this.
So, I've focused on nailing the diet and that works for me. I basically didn't exercise more than once a week so far this year, but now that I'm down quite a bit, I am naturally finding myself to be more active.
In the long run both exercise and eating right are incredibly important, so if you're overweight and want to be more active, get rid of the extra weight and you might find yourself exercising more by default.