A few thoughts on characters, specifically Darth Vader and cameras during the race as I experienced in my 2 runDisney events and only 2 1/2s in my life.
For the 2011 Disneyland 1/2, I was so consumed with finishing that I elected not to worry about photos along the course for fear that the time would be too costly. I trained to finish in roughly 3:15 give or take and was in the last corral. When all was said and done, I finished in 3:05 with no stops whatsoever.
For the 2012 Disneyland 1/2, I was much more confident about finishing and after talking to some fellow runners at the Mousetalgia meetup who commented that character photos were a nice distraction from running, I felt like I had some time to devote to character photos during the race. In looking at available characters on the course, I decided to stop for Prince Phillip and Princess Aurora in front of the castle and Darth Vader. The combined line for both took between 10-12 minutes, with Darth Vader being the longer of the two lines. With character stops, I finished this year in 3:07:51.
Because I'm not lugging my DSLR around during a 1/2 marathon and since my cell phone is over 4 years old, I opted to rely on Brightroom's race course photographers. The photos with Phillip and Aurora turned out nice, but Vader and the stromtroopers have yet to show up. Honestly, I can live with that since I met Vader previously at Star Wars Weekends and the cost of Brightroom digital download is $35 per photo with a $99 special for everything, including video in Angels Stadium.
So trust your training. If you can train at a fast enough pace to finish and have some time left for characters, I say go for it, especially since Vader is not available at all outside of Star Wars Weekends. But if you just want to finish, then remind yourself how awesome it will feel to get those coveted character photos in the parks after the race with your finishers medal around your neck.