Nutrition on an ultra marathon

[Deleted User]

I'm doing my first ultra on 6th of October - London to Brighton (54 miles). I'm planning to have 500ml of isotonic drink and an energy gel at miles 10, 20, 30, and 40. I'll carry it in a bottle belt. I'll just have another isotonic drink at mile 50. At every 5 mile interval there are cups of water provided.

Does that sound about right? If there are any experienced ultra runners out there can you tell me whether that's likely to be enough?