It going to be like staying in a non-host resort, you are going to drive to the start. You need to look at the race instructions for the transportation window and get into the lot well within that window. If you run too late you will not get into the parking lot.
As a generalt rule you will not want to follow the 'normal' GPS driving directions that lead you through the intersection of buena vista and Epcot Blvd. You will want to go around and enter Epcot parking from World Drive.
We stayed off property both times for the Princess half. Once in the Doubletree near downtown Disney and last year at another hotel. It was fine staying off property. I just followed the directions that were given in the packet to get to Epcot on race morning and had no problems. I was able to leave later than the last bus from the host hotels and still had plenty of time.
I work for a hotel company so I take advantage of my discount and have had no problems driving to the start. Saving the extra money was worth it because I am cheap! We passed all the buses getting to the start line for the Princess. I have heard that the WDW Marathon may be a bit different...but we will see.
Just a suggestion - we're staying at Bonnett Creek Hilton after the half next weekend. I noticed on their website they're offering packages for Princess - they're doing the early morning transportation, a pasta dinner, etc.