What Causes Tiredness On Walking For Short Distances?

I am a female 56 years old. Good condition as far as health and see physician regularly. I have noticed that when I walk I become weak and when I get back from walking if I eat something this will go away. I have put on about 10 additional pounds this winter and I need to get this off. Is this anything I should be concerned about or continue to walk and try to lose the pounds.