I thought I was in fairly good shape-I knew I needed to lose around 30 pounds,but when we got the elliptical I can only do 15 minutes before my legs get really tired.Is this just because I am not used to the machine?I was walking daily for around 30 minutes and doing the "my fitness coach" for the wii.I would like to be able to work up to an hour daily,but I do not want to hurt myself.Any tips for using the elliptical?Thanks