I did SITS last year – first solo 24 and came 8th. My plan was to eat proper food interspersed with the typical energy food. Not only for energy, but for something to aim for.
Typically, for fluids I alternated between energy drink, hydration tabs and water
For food, I made sure I had a little every other lap – gel, banana, mini home made cheese and potato pasty, thai sticky rice balls with soy sauce etc. Real mixture of sweet and savoury but something I could just grap and go/eat on the move.
Every 6 hours I’d stop for something more substantial depending on how I was feeling – including a full cooked breakfast at 5am to keep the spirits up!
I felt fine throughout the event in terms of energy – knee pain was a different matter. There are ‘problems’ which can arise beyond 12 hours which you will not have encountered before. Try to fix then as soon as you notice them otherwise they can make things miserable!