Good for you! It is definitely a different sport, much of which is mental.

That said, I have only done a 6 hour solo race and I finished 5th out of 5, but I won! Like Hannah said, plan your own race unless you're competing to win it. My goal was to see how I would do in a 6 hour race, what foods would work, rest patterns, etc, and just to say I could do it. Turns out it was a wicked hot day and I don't do heat well. It was also a short lap - only 4.5 miles. I stopped briefly after every lap and in retrospect, I should have stopped after every 2-3 laps. Stopping made me want to stop riding. My goal was to ride 6 laps and I did 7. I found food didn't sit real well but sport beans and gels were alright.

Make sure you have good lights and backup lights. If you can find a person or two for a crew all the merrier. Find a place before the race to set up your cooler so you don't have to go too far off the course to get there and waste energy. Most races will designate an area for solo riders to do this. Go at your own pace. You will most likely be on the course with teams and shorter distance racers so don't get sucked into ramping up the pace to keep up with them. Remember to take your electrolytes and practicing using them ahead of time.

Make sure you bring a cheering section with cow bells for motivation - really, it works! I would recommend Mountain Biking Like a Champ (or something like that) by Ned Overland. Most of all, have fun, take pictures and post a race report!