I think it sounds great. As we age, our joints protest added weight more than they did in our 20s, for example, so your knees will thank you every time you take the stairs. Dropping 20 lbs. is perfectly appropriate. I am 5' 6" and look like those monumental carvings on Mt. Rushmore if my weight reaches 145 lbs. -- everyone is different at where they look best and feel best, but in my case it felt even worse than it looked. So go for it, talk to a doc and start running or going to a gym. Good luck!
I don't believe it would be too small a weight to be healthy for you. I would say just talk to your doctor about your nutritional needs and your weight-related goals and what exercises you are physically able to do, and see what happens. At the very least, you will be a healthier you even if weight loss does not happen. Good luck! Losing weight is hard at any age.