We've ticked off some of the basics already, next we need to think about the actual race plan. Now some people turn up and have no idea what they're in for and yes, do get to the end. My best performances and those of my clients have come from having a race plan. It might be as simple as just knowing a few basic things like the number of climbs in a race or which parts of the course you expect to do during the night.
I like a balance between simplicity and detail although it depends a little on the route... For flat races or routes that are lapped perhaps, this is easier as the terrain is consistent... For mountainous races which I like, you might have a couple of hills in the first half then 10 giant climbs during the second, either way good to know this and plan accordingly.
So you should have an idea of what sort of pace you can move from your training and warm up events. Eg my 3rd 100 mile race with about 4500-5000m ascent I had done a 56 mile race with about 2500m ascent and a 40 mile with 2000m at an intensity that was comfortable for me. I figured if I could do 56 mile in 12 hours relatively comfortable and with some navigational errors probably costing me 30 mins total I'd be able to finish the 100 mile about 24 hours all going well (In reality I did 18 hour 23 as I'd underestimated my fitness and pace and over estimated the course!!)
Knowing approximately what to expect time wise, even a ball park, helps you decide on the kit choices (step #4) and fuelling strategies (#5) You might love detail and do everything down to the minute, You might decide that the 2nd half of your 100 mile is going to be about 2 hours slower than the first half, then when you get halfway you know what you're facing.Not having a plan is like setting off driving somewhere when you have no idea of the destination and can lead to complete confusion or overwhelm.
Sure you might hit a bump in the road and have to adjust your plan as you go but at least you can prepare mentally for this.If you are planning a 20 hour finish time and you hit half way in 12 hours, you can realise that you're 2 hours off the plan and decide an appropriate adjustment... Maybe you just massively under estimated the route and potentially could be out for 4 hours longer than you expected, now you can start fuelling additionally to cover it or pickup extra kit from your drop bag because of the cold night you're about to be running in unexpectedly...
Good Luck :)