a diet is like a marathon
According to running-training-tips.com, starting a marathon too fast is the best way to ensure you'll walk across the finish line, if you're able to finish at all. They advise to start even slower on race day than you've trained for, and to pick up your pace, slowly and carefully, once you've crossed the halfway point; doing so should enable you to pass hundreds of people during the last 6 miles of the race.
The start-slowly concept holds true for health, fitness, and weight loss, too.
If you start your program with a super-restrictive diet, one that's also a complete change from what you're used to eating, and add in an hour of exercise per day (when you're used to doing zero)... Odds are you'll quit within a week, two at the most.
Better to have modest goals, look more to the long term, and start slowly. Trying to lose a ton of weight too fast is a recipe for frustration (and overwhelmedness. - yes, I made that word up.) Instead, focus on losing 10 pounds over the first 6 months, and 25 over the first year. This gives you time to ease into better eating habits, and allows you to start with smaller chunks of time for exercise.
If you've been consistently gaining weight, your first victory would be to get through a month without a weight change. Then, maybe the second month you're able to lose a pound. Yes, that's only 1 pound in two months. But it's a gigantic turn-around from where you've been - now you're heading in the right direction, eating better and developing the habit of exercise, and you can begin to pick up your pace.
You can always adjust your goals higher once you get ahead of schedule.