The main tricks/techniques for completing a race or course are
- knowing how to pace yourself for the whole distance so you don't burn yourself out before halfway.
- knowing the course so you know what's about to come up and be ready for it. IE have a gel 10 minutes before a climb starts, or where a specifically steep grade might surprise you, or which corners you can take faster and which corners have to be a bit more restrained.
So the best training you can do is to ride the planned routes before hand. That gives you all the knowledge you need of the course.
If the route is a one-way trip then consider riding the first half then turn around, and some later week drive to the end, ride back to halfway and finish the rest.
Riding the course the wrong way is almost as useful as riding it forward.
Since you don't have a lot of experience with gravel, you need to practice ON gravel or similar. Try and find some road or track near you that replicates the conditions found in the race route. There's little point riding sealed road or mud if they are not like the course.
That said, if the weather could make it poggy, having mud practice could be beneficial.
Try and get at least one ride in each weekend, and an endurance-style ride in the week (perhaps on the way home from work?)