Hi Rusty,
I shoot our local athletic clubs Trail half marathon regularly, mainly for fun and because I have friends competing in it. I don't know exactly what type of shots you are looking for but a couple of things that have worked for me are as follows.
I try and shoot the start of the race and then get as many shots of individual runners as I can. People love having semi decent shots of themselves competing. Because I am not the official pro photog I don't tend to worry about getting them as they finish, I am not treading on anyone's toes then.
To that end once I have shot the start I find a place nearish the finish of the race where the pack has pretty much spread out and so getting shots of individuals is much easier.
When I pick my spot I want it to be in the full shade so I don't have to contend with harsh shadows or blown out highlights on sweaty skin etc. Dappled light is a PITA too.
I like to be able to see a good way ahead. This helps especially when a clump of runners are approaching so I can estimate in which order I will try and shoot them, individually or in a group if they are keeping together.
I pick a target mark on the ground where I will start shooting once a runner crosses the mark. Or earlier if there is more than one runner together so I am not panicking trying to catch everyone if they get too close.
I always shoot in burst mode and take 3-4 shots of each runners. This means I should always have some nice and sharp shots of everyone. It also gives me more leeway to pick the most dynamic shot. I also try to make sure the runner is not looking terrible as Jerome suggests (tho sometimes that is really really hard after a long run).
Oh and having a monopod is awesome!
Hopefully this is of some use, and good luck with the shoot.
Here is a link to the last lot I shot so you can see the results that I get with these techniques.
http://www.flickr.com/photos/94681368@N00/sets/72157635146674571/
Cheers
Angus