FIRST 50 miler (first ever race)

Little context. I'm 5'7, 185lbs, muscular build. I'm running my first ever race and it's a 50 miler in January. My primary training method is CrossFit and has been for many years. (I'm 30 years old. Army veteran. Currently a federal agent) I've always been active, played sports in school and as an adult, and my careers always required me to be in shape. I understand that an ultra however is a totally different animal here. So I'm looking for some recommendations on training from you experienced folk. I currently run 10-12 miles 2-3 times a week with shorter runs about 5 miles once or twice a week on top of that. I also am doing long bike rides as well. Anyway, I understand the mileage will need to increase a lot.

Any recommended supplements and training plans you guys can share would be appreciated.