No backing out now

I have become completely obsessed with reading about ultra endurance mountain biking. In particular, the Arizona Trail Race 300.  I read a different blog from the people who do these crazy multi day events each night. Im trying to soak up as much knowledge as I can before April 19th. I have committed myself to doing the shorter version, 300 miles. the longer version is 750 miles, out of mi league.
I have had this on my mind for almost a year. In December I got serious about my training and started doing really long rides. I got some help for my ongoing back issues. I do exercises  3-4 nights a week - on nights I don't ride. I try to eat healthy, and have even consulted a dietitian. I am as fit as a 45 year old man can be. A man with a family and full time job, that is.
My goal is 4 days. I hope 75+/- a day is realistic. I will carry all my gear, sleep system, food, tools, clothing (hot & cold weather), light system for night riding, first aid, ect.

I just booked my flight... so I guess theres "no backing out now"