The point of the other poster was that you won't HAVE to go to HS that day...there are busses from all of the resorts to/from the Expo for marathon weekend. They may pick up in a slightly different spot than the regular park busses...should be signage or the bell services people will know.
Just to add, the buses they use for Marathon Weekend are not "Disney" buses, they are chartered buses. There are buses of these type running from every resort to the WWoS for the EXPO. Just ask the front desk of your resort where they are picking up from if you don't see the signs for it. There will also be signs at the WWoS for where to get your bus back to your resort.