Wimp! Just kidding...
It's probably more important where the rough stuff is in the marathon course, than if there is any at all. It's good to have some at the beginning to remind us to run with gentle technique from the beginning, but like a nagging friend, can be pretty annoying when we've run 15 or 20 miles and are getting a bit tired, and still have a good distance to go.
However, if you can't run on the rough stuff without too much discomfort, at least for some short distances (I don't know how much rough stuff is in the Ojai to Ventura Marathon, so I'm not trying to pick on you in particular, or that course), you may want re-consider if you're really ready to run a full marathon.
You can probably get away with running on the sidewalks (be sure to run across all the timing mats, which will likely be in the street), but if you're winning the marathon, you'll want to stay on the official course to be sure you don't get disqualified. Also, the crowds may not notice you're running the marathon, and not only will be in your path much of the time, but won't be cheering you on (which really can help boost your energy, especially near the finish line).
Remember, if you run your first marathon on more challenging terrain, you'll probably not find another marathon that won't be easier, including that same marathon the following year ... unless, of course the asphalt erodes even more during the year between the two events.