That's the problem with diets. The first few weeks are great because you lose pounds and pounds. Unfortunately, it's not sustainable because it's not weight you're losing, it's just excess water and shit that was clogging up your digestive system.
The annoyingly simple fact is 1lb of fat = 3500kcal. And it's so much easier to eat that number of extra calories than it is to get rid of them through exercise.
Bog standard Curry/Poppadoms/Rice/Naan and 4 or 5 pint night out. 3500kcal.
That's about the same amount of energy as you'd use up on something like the Dun Run.
Want a lose a pound a day? Simple. Do a 180km bike ride but don't eat anything more than the 2500kcal you'd normally eat in a day. Sustainable? No (unless you're super-fit).
What does work is:-
Stopping eating the wrong foods, and/or too much.
Moving your meal times around (big breakfast, average lunch, small dinner) helps by increasing your Basal Metabolic Rate.
Doing more exercise helps by using more calories (obviously) and contributing to a higher BMR.
Training your fat metabolism with regular exercise so that you can do low/medium intensity exercise and not feel starved.
Not going too fast. You'll enjoy it more going at a moderate pace and you won't just use up your glycogen stores (which aren't so easily replaced by burning fat).
Serious illness works because your appetite disappears and your BMR goes through the roof whilst your body fights the infection/otherwise heals itself. It shouldn't be a long term strategy though.
1 hour of moderately paced exercise a day = 3500kcal a week = 1lb of weight loss a week. Sustainable yes, but overcompensating with food intake will ruin this. I'm back at this stage. I've just had a 1100km month on the bike and haven't lost a pound.
Eating 500kcal less a day = 3500kcal a week = 1lb of weight loss a week. Very hard to sustain.
Learn to love the steady progress of 1/2lb, 1lb, 1+1/2lb or sometimes 0lb, a week loss and keep at it.