It depends. How do you feel after running 3.5 miles on regular flat surface? If you feel a little exhausted, you may want to consume more carbs than usual before the event. Not only are you running on mud/dirt, but you're also going to be carrying some of that mud/dirt on you as you run.
When I ran my 10K this past weekend, it was on a very hilly dirt trail. I had a Chipotle burrito (~600 calories) in the afternoon, then a banana and a Snickers bar couple hours before the event. I was kinda burned out halfway through. Luckily I had some Gu and that helped me get through the rest of the race. If the course was on a flat surface, I probably would have had an easier time finishing.