Aerobic exercise is any where you breathe more heavily and the heart gets going. Also know as cardiovascular, it's through endurance and activities that involve stamina that gives you the most cardio benefit. Typically these are exercises that can be done on a treadmill like brisk walking, jogging and running.
Edward R. Laskowski, M.D. at The Mayo Clinic says adults should do 150 minutes of moderate exercise per week. This is brisk walking so that you feel slightly out of breath and 150 minutes equates to 30 minutes per day, 5 days a week. That is quite a lot. If you jog or run on a treadmill, that counts as vigorous exercise and then you only need to do 75 minutes per week.
You won't be able to run or brisk walk 15 or 30 minutes a day straight away. It takes time to build up your endurance. Losing weight will get easier over time. I have created many workout schedules that help you lose weight and slim thighs by burning fat. One very common routine is the couch to 5k. The link takes you to the treadmill version of the couch to 5k. This is a walk run method to get you for jogging 30 minutes, gradually building up, over 9 weeks.
Here are some of the routines I am referring to. Remember a routine to lose weight will help slim thighs as well. I've got mainly beginners to give you an idea of aerobic exercise whether you choose to walk or run:
Include the diet too
When you start exercising to lose weight you get results faster by changing your diet as well. I recommend you don't eat a very low calorie diet because you need calories to help fuel your exercise. Eat 3 well proportioned meals and maybe a few light snacks in between. You're best not eating a main meal within 2 hours before exercising. Eat right away afterwards if you feel you need to.
When I lost 30 pounds I ate the same as I always did. I know I didn't eat too much really, I just needed to get some exercise. I sometimes felt hungry but as I was eating 3 meals and a couple of snacks a day, I knew the weight will come off and it did. When you get to a point when you're at your desired weight or your thighs are nice and slim, slightly raise your calorie intake so you maintain the weight.
My best advice is just to begin. Whether you can only walk or run for 5 minutes at first, that's ok. If you need to build up slowly then do that. As long as you do build up because your body will change. You can naturally run or walk that little bit further each week. That's how you build endurance and stamina and the weight will come off. Don't worry about having big muscles, that won't happen from treadmill exercising.
You will soon see slimmer thighs and I would weigh yourself every week at the same time and day. Your progress will show on the scales. Do the thigh exercises if you want, but a treadmill is the way to go to get visible results. Any aerobic activity done to a good intensity to you get slightly breathless for a period of time will work.