I'm going to have to respectfully disagree with you Cruxis!
You will be working out forever if you do a heap of light weights at 20-30 reps.
Aim for between 10-15 reps, and do three sets when you first start. Start light. If you find it easy and you are doing the exercise correctly, up the weight. At first it is always a bit of trial and error. And look at exercises that work out more than one part of your body and focus on major muscle groups, such as squats. Combine is some body weight exercises such as push ups and you'll start to notice a difference in your strength pretty soon.
You will not build a heap of muscle lifting weights this way, you will tone and get some definition. Women start to lose bone density and muscle mass from the age of 30, so weight bearing exercise is really important as we age. Plus, thanks to our hormones it's pretty hard for women to get super buff- unless you live on a diet of poached chicken, protein shakes, egg whites and steamed broccoli.
If you are looking to lose weight, getting some muscle is the best way to do it! The more muscle you have, the more energy you burn.
You will also find that adding some weights will help you perform better in your cardio as well.