No (or low) carbs is a pretty common dieting technique, originally made famous (at least where I am) by the Atkins Diet. My simplistic understanding of it is that when carbs are not available, your body burns fat instead, starting with the fat you consume and then if necessary the fat that your body has previously stored (hence losing weight). A colleague of mine does this for a month once a year to get a bit fitter ahead of his scuba diving holiday, and it works wonders for him.
Regarding long-term use of this diet, it would seem that opinion is divided and I've no idea which side is likely to be right - http://en.wikipedia.org/wiki/No-carbohydrate_diet is probably a decent place to start if you want to find out more. But certainly, while it's suggested that long-term use may not be a great idea, there's nothing to say that it would cause problems such as not being able to exercise for two hours a day.
Regarding long-term use of this diet, it would seem that opinion is divided and I've no idea which side is likely to be right - http://en.wikipedia.org/wiki/No-carbohydrate_diet is probably a decent place to start if you want to find out more. But certainly, while it's suggested that long-term use may not be a great idea, there's nothing to say that it would cause problems such as not being able to exercise for two hours a day.