I plan to run the Army 10 Miler. What kind of weather can I expect in Washington DC in October?

From my two year experience, the weather is little cold like in the 60s but it may change constantly to like 70-80 other then that is a great experience. I will be there this year as well.

Last two years, clear and in the 40's while waiting to start.feels great after you get warmed up.

You never know, one year it is Hot, the next it is cold! I would count on mild.