I think AZ means that he burns a high percentage of fat (as opposed to carbs) at moderate outputs. I do 'bonk rides' everyone once in a while (once or twice a month). I get up in the morning and have no calories (I still have my black tea) and only water with me. I then go ride 1-2 hours at anything from an endurance to SST pace. I do this to get my body to remember that it can just burn fats if it needs to. Doing one of these rides every day would not be good for me overall.
50 miles a day is completely doable, but it won't initially be easy. There will be a time from the end of the first week to, maybe the third week where your body is getting used to the miles. Once you get acclimated to it, though, you'll be dialed.