I know Canon makes a AC adapter that fits into the battery slot that provides constant power and eliminates the need for rechargeable batteries when you have an outlet around.
If you take 1 picture per second * 60 seconds per minute * 60 minutes per hour * 2 hours I'm pretty sure you get 7200 pictures.
If you can set up your exposure and everything before the runners show up (based on the current world record you should have nearly 2 hours to set up if you start when the runners start running) then you could try to shoot in JPEG to reduce the file size. I don't image that you would want to process 7200 RAW photos except to batch white balance or something and you can adjust that with some prior thought for the JPEGs, except a cloud will probably pass and screw things up. So there's a tradeoff there you'll have to consider.
Then consider that you can get 128GB SDXC flash cards now. Depending on your camera and it's configuration this can be A LOT of photos.
Then all you need is the intervalometer and other equipment that dpollitt suggests. Though you might also try to use the intervalometer a little more skillfully. There will be times when people aren't crossing the finish line so if you get a long enough cord you can start it as people approach and temporarily turn it off when they finish until the next runners get close. This could dramatically reduce the number of shots, but requires you paying attention.
If you wanted to get fancy (and you aren't concerned with the specific timing of the photos being of them crossing the finish line precisely) then you could try to tie into system that tracks runners' times. http://bitshift.bi.funpic.de/en/dslr-remote/hardware.php shows some hardware solutions to connect a headphone out port on a computer (in this case a phone) to a wired or wireless system triggering system. You'd have to program your own utility to utilize the adapter, but the advantage would be that you would have FAR less pictures at the end of the day and you could link the images to the runners as they are created. And if you have fewer pictures you can do them in RAW which would help with the WB in the event you have inconsistent weather (clouds coming and going). If you did create such a system (and one does not already exist with a strong foothold in the market) you could probably sell it or at least leverage it as you market yourself to photograph other marathons.
While I'm thinking about it, you may want to put a grey card in one of the corners of the frame (might want to make it big-ish in case someone steps in front of it). That way you can easily adjust the WB and then crop the card out. Some of these steps may involve some contacts within the event committee, but it would help get better shots.