I'm new on this, well really a week old...

Ok so today I signed up for my first 5K Mud run it is in December so I have time to get ready. I need to drop about 15 lbs and firm up. I'm getting married next August so this will help me get in shape for the wedding. I eat pretty healthy, borderline raw foodie. I'm 52. So any advise on this Mud Run would be great.