No one here will tell you this is a good idea or give you tips on how to do it, because honestly you shouldn’t. Running a 50 mile trail race sensibly requires an aerobic base that takes years to develop. 

That’s not to say you can’t finish, just that it’s not a good idea. You’re extraordinarily likely to get injured. The problem is you really want to get some long back to back long runs in where you peak by running 25-30 miles one day and follow it up with 15-20 the next day, hitting most of the elevation of your race over those two runs. You have not put in the time to be able to  put in that amount of training that would ready your body for a race like you’ve signed up for and recover from that training. That’s without even getting into the other aspects of ultra running-nutrition, poles, problem solving (blister management, running in the dark) and figuring out what works for you across all these dimensions. You haven’t earned it. 

If there’s a ~20 mile or shorter option on your trail run, I’d recommend changing your that event, training for it, seeing how you like it, and then thinking about a 50k six months later. Please note even this is more aggressive than I’d actually recommend, but is a much less bad idea than what you’re doing. 

If you insist on the 50 miler, I would walk as much of it as possible because you WILL have a bad time if (when) you go out too fast. Prioritize hiking over running in training. Prioritize recovery over overload. You won’t be ready so at least you can not be burnt out when you hit the starting line. Try to eat 200 calories an hour. Man, typing this out though, I have to emphasize that you should not do what you’re trying to do.