Planning for the event
So I break down this planning part into 3 pieces course lay out, elevation profile, and aid stations. My first couple of trail races these items didn't really come into play because I hadn't been told about them; but when I did my first trail marathon in the toe of Virginia I really wish I had known about these different aspects.
Course lay out deals with the lay of the land. What direction do you go? Do you have to change trails during the race? Are all the questionable intersections going to be well marked? I've found that one of the best solutions to all of these questions is to be trail savvy, which is to say can you follow trail blazes? Be able to read a trail and sense, figure it out, which way the trail goes can mean the difference between getting lost and not finishing or finishing. Don't ever, ever take for granted that the person you are running with knows where to go. One way to test how savvy your are on trail is to go to a trail you've never been to before and with just a bit of study of a trail map go do a long run and not get lost.
Elevation profile is a big deal and you have to be able to figure out if a race director is 'over stating' the elevation gain. I've run across a couple of directors who will make a statement of '40,000+ of elevation gain loss', so the gain would be about 1/2 of that. Elevation gain tells you 2 things, first how much climbing you have to do over the course of the race. Plus it tells you where all the climbs are. When you look at the side profile of the race you get to see how the climbs come during the race. Are they just in a few big climbs or a bunch of small ones? This will help drive some of your training. Will you need to be able to climb for a long period of time? Will the race go up and down for a long time? Understanding what the elevation profile tells you about what you need to be able to do will go a long way to you finishing your race.
Aid stations are keys to your nutrition during the race. This is the place to eat and drink during your race, but don't linger. Will they have what you want? Will there be anything left when you get there? Knowing what food will be at an aid station will help you figure out what you should carry with you during the race. If your race allows drop bags it will help you plan what you need when and be able to leave it in a drop bag at an aid station for when you get there. Aid stations for me are about 2 things food and drink. If they won't have what I like then I have to plan, figure out a way, to have those things when I need them. Get in and out as quickly as you can is a key.