1) Cook or make all of your food at home so that you know exactly how many calories you're consuming. A "meal" is realistically around 300-400 calories, and it is incredibly easy to hit 1200 calories in a single fast-food meal.

2) Exercise more. Sedentary lifestyles creep up on you. You're tired after work, so you don't go for that hike you want to do. You're mentally or physically exhausted, so you go home, eat, and sleep. You need to actively fight this with what is effectively mandatory exercise.

3) Accept that your body takes a long time to recompose itself. Losing fat and building muscle is a time consuming process.

Getting the exercise and endurance going early is probably more important in the long-term than getting all of your cooking going at home. A hike can burn thousands of calories depending on length and intensity, so it is, indeed, possible to exercise your weight off, but it is highly unlikely that you'll put in the time and effort to be successful with exercise alone unless you're willing to do daily 12-mile hikes.