The treadmill is an excellent option for walking and has the benefits of variations that you might not find in the outdoors. First, it works just as well when you cannot access the outdoors for reasons like cold or pollution. Secondly, you can time your walk, use the motorised treadmill to increase your pace and keep a tab on the exact number of kilometres/miles you have walked. What it cannot do, however, is give you the exact caloric loss (unless you have one where you can enter your weight).Having said that, the non-motorised treadmill, though cheaper, does have as good benefits as the motorised one, without the above variations. I would recommend the one, which is on a flat bed rather than on rollers. There are models that also have variations in the incline of the treadmill.The duration and speed would depend on your present fitness level and your goal. For fitness benefits, even a small 10-20 minute walk on it can make a difference, but if you are looking to lose weight, you may want to spend more time on it. As long as you are comfortable on it and can slowly increase the speed over time, you are doing well and benefiting from the exercise you are getting.Please put on your walking shoes when you walk on the treadmill to give you enough support and traction. The effort involved with a covered foot will be less and the impact in the body felt will be drastically reduced. The impact going through the foot is partly what makes you feel the pain in the legs. Adjust the incline occasionally to see where the pain is felt the least.You must also make sure you warm up the whole body before going for a walk, whether outdoors or on the treadmill.As regards reducing your weight effectively, any aerobic activity like what you are doing now, will help you reach that goal. But along with that, you have to couple your efforts with a sensible diet to aid the process. And the fat stores from all over the body will reduce, not just from your waist. But exactly how much is very difficult to say. All I can definitely say is that the effort is well worth it. Even though you may not be able to see the results immediately and visibly, exercising is beneficial to your overall health. So let that be your ultimate goal.