I would say 2 months is not enough time to train for a 30 unless your horse is already in pretty good shape, since it's your first ride.
However, this sounds like a great first ride to do because the trails are dirt so you wouldn't have to worry about bruising your horse's soles. It also sounds like moderate terrain so it's not straight up the side of a mountain.
So IF you were dedicated and started training tomorrow, you could possibly do it. Or you could call and see if the 30 mile course is done in 2 loops, then you could just do the first loop and see how your horse feels before doing the whole ride.
The first month you would have to work your horse slowly up to doing fast 5-10 mile rides (on trails, not in an arena).
You would need to be able to have your horse do a 12-15 mile ride (mostly trotting and cantering) at least once a week for the last four weeks before the ride. Plus two fast 5-6 mile rides a week that last month.
If you can, practice having your horse stand after riding while you or someone else checks him over (to practice the vet checks). They will listen to his heart, pick up his feet and listen to gut sounds. Also practice trotting him out in a straight line.
The workouts I am describing would be to help your horse FINISH, not to win. In endurance you shouldn't think about trying to place for your first several rides. To finish is to win.
Here are more details about the Grizzly Mountain ride: Endurance Riding Online - AERC Ride Detail