I’d say make sure you can do a solid repeatable 8 hours in the saddle solo.. That will teach you whether your bike set up and clothing is suitable.
Also when to stop for a rest, what sort of food and drink you need, too little and you won’t finish, too much and you’ll be hauling more weight which adds to fatigue.. Taking into account feed stations where you can replenish liquid and food.
And lots of hill climbing practice!
And the obvious.. Ensure your drive chain is running nice and smooth, gears shifting nicely and brakes are working well with plenty of pad life left.
I tend to struggle to eat properly, as in by the time I feel like eating I’m too far gone in terms of I’ve got no energy left so I have to sit down for 20 mins, force some food down and wait for it to start reaching my muscles lol!