I don't run, but my husband has done four organized 50 mile runs and 5 or 6 on his own. He had one DNF and one end of run super bonk, and has used those experiences to figure out what works for him. He had his most successful run this spring in the Old Pueblo 50, and ended the run feeling really great. In fact the organizers told him his was the best finish they had seen!

I'm sure his advice would be:

Eat, stay hydrated and keep your electrolytes up! Eat 200-300 calories per hour and drink based on your sweat rate. Don't push yourself and rest/walk when you need to.

Good luck- and my wish/hope is: I hope you don't lose too many toenails!