Starting line?? wow, you are very nice to get up early & wish him off w/ PD
At the drop off point at the start in the EPCOT parking lot--there is a "staging" area where people can drop off belongings, buy food and drinks, bathrooms, a band playing, etc....around 5am they open up the path to the start--this is maybe .5 mile walk to get to the starting corrals. This is for registered runners only, you will have to bid your BIL farewell at this point.
I recall seeing people cheering around the start--on the sides of the road. It is DARK DARK DARK at the start, very hard to see your friends/family!!
Are you planning on going other places that morning to cheer? My advice is to take the monorail! My family got stuck in terrible traffic that morning and didn't catch us until mile 24 on the boardwalk!
The Boardwalk is a great place to meet up--anywhere along the route from MGM to the World Showcase entrance to EPCOT is a good place to camp out and cheer, esp if you want to avoid the finish area. The finish is set up w/ medal bleachers for spectators. If you have park tickets that you are going to use that day--EPCOT has many good areas to cheer as well. One of my favorite areas was under the arches--they have cast members waving the flags from the countries as you run thru the arches, it was GREAT! Also, as you exit EPCOT there is a choir singing which is absolutely awesome.
If BIL is going to meet you all after the race on the Boardwalk, w/ a park ticket he can enter EPCOT and walk thru to the World Showcase entrance and voila, out on the Boardwalk. We did this last year and it worked out fine--got to see the runners still on the course and get rid of some lactic acid build up.
If you all aren't going to use park tickets the day of the race . . . . . there are buses running after the race to get runners back to resorts and of course the monorail will be running.
Hope this helps! I thought I had everything figured out for my family last year and sending them to the spots designated by Disney in the info packet was the WRONG move! Maybe this year it won't be as crazy race morning w/ the marathon and half marathon on different days!!!