I do several nets like this a year for various races. I'm sure you will pick up what you need
to know on that first day while stationed with the experienced operator, but here are a few ideas.
Your lead for the net should communicate to everyone an operations (ops) plan. This
plan should include frequencies to be used (and backup frequencies), what traffic is required from each station, times each station are expected to be open, etc. If you would like to see a sample of the plan we use just drop me a note.
I've done some races where each station just called in the first runner (male/female), and then when the last runner comes through. Other races we called in each runner (in batches) to net control. Of course any emergencies would be radioed in immediately. We've been fortunate to only have a very few emergencies on these events. Most traffic is of the form, "Can station 2 get more ice/water/Gatorade?". "Station 2 what time did runner 4 pass through?" etc.
I find public service to be one of the most rewarding aspects of Amateur Radio. I applaud you for getting involved so quickly after being licensed. If this is something you find you enjoy you might consider joining up with your local ARES group. Info about that can be found from your Section Emergency Coordinator, or Section Manager.
Section list is here: http://www.remote.arrl.org/sections/
Click on your section to get contact info for your Section Manager.