People hold body fat in different proportions throughout their body. Weight is not a gauge of your aesthetics or overall level of fitness. Some body builders at your height weigh over 200 lbs but their body fat is incredibly low so they don't look fat.
If your goal is to have a flat stomach and thin thighs, plain and simple you have to reduce your body fat percentage. You can't reduce body fat in one area and not others; the human body fat scales proportionally. This comes through consistent dieting for a long period of time and frequent exercise. The body's response to this is also different for each person. Some people can very easily loose body fat with minimal effort and some people have to work harder but it is never impossible. So in general don't get frustrated if you don't see results in three weeks. It usually takes about eight to really notice a significant difference.
In reference to how long you have to exercise, that really depends on your health level and if it is even safe for you to engage in it. If you are in relatively average shape start with 30 minutes a day. Then as it gets easier through the weeks add more time or intensity (try working out harder). Keep changing your routine every few weeks (try running outside when you get bored of the machine or doing calisthenics if you can't get to a gym); your body will become accustomed to doing this type of exercise very quickly and your results will start to lag. Most importantly, challenge yourself. Your body won't change if it doesn't have a need to; create that need through your intensity and intervals of training. The main thing is diet, however. You can work out for 10 hours a day but if you aren't eating the right foods consistently every day, week after week then you won't see any change. Good Luck, Hope that helps and feel free to ask any questions.