Your mind is your best friend when it comes to achieving goals. You have to believe that you CAN do amazing new things. The mind is very powerful and can either be a tool to help or hinder us. I believe that running is at least 50% mental. Chances are that your body can go the distance, if your mind doesn’t talk you out of it.
One of the “secrets” of successful athletes, business people, and politicians is that they have confidence. You have to eliminate the negative self talk that goes through your mind frequently and replace it with positive truth. Get rid of “I can’t” from your vocabulary. Surround yourself with positive, encouraging people as much as possible.