I have volunteered at Marathon weekend on numerous occasions, always during the half or full. I have done water stops (behind Splash was fun!), food stops, and crowd control. The main draw for me--which has since been changed--was the immediate receipt of a one day park ticket (for those volunteering for half/full).
Registering for volunteer shifts has become as crazy as registering for the races. In the "old days", you could peruse the list of shifts and take at least some amount of time deciding. No more. Also, there are fewer shifts for individuals than in the past. Many stops are covered by groups. Also, at least once, I had my volunteer location changed.
It may sound like it's a pain, but actually, I enjoy volunteering for races (not just Disney) I have frozen my tail off (2010 half), handed out a ton of water, watched a bus get stuck on the median shortly before the start of the full (that was a trip), and run out of gels at a food stop (NOT cool on Disney's part). But it's still fun to cheer on the runners and fun to be able to get behind the scenes at several of the parks.
So as mentioned, if you want to volunteer, do it on Day 1 as soon as it opens.