Don't push yourself too hard during the ride, just take it steady.
Pick your battles,for your first 600 find the easiest, flattest one around.When you've done one you will have much more confidence.
Get plenty of sleep in the week before the event.
Lay off the booze for a couple of weeks before you attempt a 600k event.
Break the ride up into sections between controls. i.e. "only another 40 miles and then I can have a rest".
Eat and drink every hour or so.
Just keep moving forward. Don't faff about every time you stop.
No matter how slow a rider you are, if you keep riding, rather than sitting at the side of the road feeling sorry for yourself, you WILL finish in time.
You have, IIRC 39 hours to do 375miles. That's under 10 miles an hour. Anybody can ride a bike at 10 miles an hour.
You don't have to be a fast rider to finish a 600, you just have to be able to grind out the miles and be stoic.