To dream of a marathon represents feelings about situations you are experiencing where competitive endurance and will power are prominent. A non-stop competition or conflict of some kind. A reflection of your performance in an long drawn-out struggle requiring perseverance.
Negatively, dreaming about a marathon may represent an exhausting ongoing conflict. A conflict where two sides endlessly compete to prove they are better than the other side. A possible sign that you are in an impossible fight to win because the other side is too jealous of looking stupid to ever give up. Surviving a long conflict while hoping that the other side will give up before you do. Desperation or jealousy to outperform someone else in a long struggle.
Example: A man dreamed of experiencing a running in Boston. In waking life he was experiencing himself in a non-stop online argument on Facebook.