Let your body help you judge too. It will not help you if you make your body overdo. I walk on a gazelle in my home. My feet begin to buzz after about 10 mins, by 15, they usually are numb. I can't change that, and I can't push it, but I won't stop the work out either. If 20 mins is it, then I will do 20 mins every other day. In time, maybe I will beable to go longer, who knows! We do what we can do....every little bit helps!