I'll reply as a runner. A runner in the US, just in case it is different elsewhere.
Every race that is decently organized, including all marathons, halfs, most 15K or 10K, some 5K thats are big, etc. all have a photography group there for photos. Runphoto.com or brightroom.com for example. They sell photos for an arm and a leg (in my opinion) but, depending on their manpower, they are positioned throughout the course. I would pick one of those and browse their results. You can probably ID the lens they used from the shot. I mostly see what I assume are 70-200 lenses.
The question is, how would you sell yours? How would you get your name to the runners that you are shooting? I personally would love to have a decent running photo of me that didn't cost $20 or more for a 5x7 but other than the email I get from the group the event has do the pictures, I would never know that Joe Blow got a good shot of me at mile 18.
The challenge I see from my view, is getting individual shots of runners. It is easy for the fast people who spread out and are nearly alone. I am quite average and am always near a huge group of people. When I see the camera, I try and speed up, slow down, or move sideways to give myself space, but that is because I am vain

Most just run in the blob and you will have a hard time. For positioning, I agree with other posters. Learn the course and find a good background, where the sun will be at that time of day, etc. Bring a monopod. I have seen ladders as well. The finish line will be crowded with fans, photogs, just finished runners, finish line support crew, and the first aid people. Good luck there.
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http://www.flickr.com/photos/56328768@N00/
Brian Niehaus