You say, "I can't do this, no way". I say, "I can do this, watch me."
Well, you'd be right. You've no chance at all and might as well quit it all right now. However, you'd be equally right if instead you say, "I can do this. Watch me". With running, and any form of activity that requires endurance, a good majority of the battle is won in your head. Set for yourself realistic goals, and let nothing stand in your way. Believe you can, and you can. Believe you cannot, and you cannot.