Having sound in the hands of someone competent is more important than ever. You only get ONE chance at 99% of the events you will capture visually and aurally. You will have ZERO opportunities for ADR. There will be ZERO alternate takes, so "fixing" the dialog is impossible.
If you have been asked to document by the organizing committee you do have a little weight to throw around. As it appears to be a competition of some sort there will most probably be a PA system of some type - see if you can get a feed and record that audio to a separate recorder. The same applies to any "after" events - dances, concerts, fireworks, etc. If there is a concert, for example, see if you can get a feed from the mix console while you shoot the band.
Make sure that YOU are comfortable - clothing and footwear appropriate for the prevailing weather. Have a complete change, plus rain gear, sunscreen, extra warm/cold clothes (depending on the season/weather/whatever). Have a small "first aid" kit with pain reliever, band aids, decongestant, Q-Tips, Rolaids, Pepto-Bismal, etc. and even toothpicks and/or a toothbrush (to get that annoying piece of popcorn stuck between your teeth that will otherwise really annoy you all day and night). Have some munchies and plenty of water. Your cell phone is freshly charged and you have the contact numbers of the event folks programmed in. Triple up on spare batteries. Double up on SD cards, tapes, or whatever you use - and make sure that they all work.
Be EXTREMELY well organized. Have a check list, check it twice, then check it again. Make sure that you have everything that you require carefully packed away and easy to access. Have back-ups/replacements for everything - that includes the camera and mic. Have a basic tool kit that can be used for your gear. Make sure you car has a full tank, check the tires, etc. before you go - maybe a tune-up a few days prior?
Make sure that you are in good shape and well rested - you will be extremely active for a very long period of time.
Be entirely comfortable with your camera and other gear - this is not the time to be experimenting!!!
Have a contract. They have asked you to do this (you haven't mentioned compensation, BTW), you didn't volunteer. Make sure that they take responsibility for the safety and the security of you and your gear (i.e. there is someplace secure to leave your replacements, extra clothes, etc. and the security staff knows who you are and what you are doing). Make sure that you are fed and "watered". Make sure that you have the proper access passes, and that where you can and cannot go are clearly spelled out in the contract. Make sure that (if applicable) the contracts for the sound contractors specify that they provide audio feeds for the documentary team, or be sure the people who "hired" you understand that there are sonic compromises if you do not have such access. Have what the organizers want to capture prioritized in the contract. Make sure that the tickets, participant/entrants waiver (or whatever) have the "fine print" that lets you use the voices and likenesses of the participants and attendees without a specific release form. (Doing interviews???...) Will you be documenting the few days prior? Organizational meetings, set-up, etc.?
The whole idea is that you plan for every disastrous contingency and be sure that your ass is completely covered, both logistically and contractually.