Pacing overall is about feel. Race day caffeine, excitement, the rest and prep work that gets put in (compared to a regular training weekend) usually leaves me feeling lighter, faster, snappier, and more focused. It's taken a long time, but I owe most of that pacing strategy to my roadie days. My coach and teammates would try to kill each other on these long 5 hour training rides. We'd go all out with attacking each other in the last 45 minutes or so. The thought was "I'd rather blow up, fail, cramp, bonk, or get dropped on a training ride while testing myself and finding my limits rather than finding out in a race."
I started riding a mountain bike in 2006, in high school, after seeing a mountain bike and liking how it looked. I bought one and rode what I though was nearly every day. I eventually bought a fixed gear bike in college for commuting and really enjoyed how fast it went on a road, so then I bought a proper road bike. I got in to a local club in Connecticut who showed me a local training race. From there I was hooked, and upgraded to a Cat 4. I hired a coach (Aidan Charles of CCNS) based on a recommendation from a friend. It seemed like a big investment at first, and it was, but it payed off BIG TIME in the long term.
Tempo is Key
That's where I got my aerobic base and my endurance. Aidan had me do so much damn tempo throughout my training I was getting frustrated. But after a year or two of doing that I lost 15 pounds, raised my threshold, and now I have a great base and ability to hold tempo for a long time which is VERY applicable to endurance MTB races, and hence how I pace myself. Transitioning to MTB racing was very hard on my little roadie body. Singletrack was NOT my friend, and I swear that it still isn't. It's what I really have to work on to keep up with the other guys out there. What I have found that helps me the most is following other riders on the trail while training. I rarely like to lead a ride - I always seem to go faster and focus more while following someone fast and letting them set the pace.
Pace By Your Competition
I've never raced with a heart rate monitor or power meter in order to pace myself. My opinion on that is this - if someone is competitive and wants to push themselves, it's better to use the competition around you to have a better race, rather than go off of data. If someone is riding away from you, you have two options...go harder to keep up, or don't go harder because what you see on your computer is saying that you shouldn't. The former gives you a better chance of getting closer to the top step of the podium!
Pacing By Distance
1. XC - Usually all out for the first 2-3 mins until a few groups have broken up. Although it usually leaves me nearly completely blown up before the first lap is over, I've always found that once I dial that back and let any stronger riders go, I calm down and recover, then I feel fast for the rest of the race.
2. Marathons - Those usually start quite hard as well but I never try to get in that all out sprint. I feel most of those starts are around or above threshold pace, include several small sprints to pass people or keep a position, but never have that 1-2 minute leg screaming effort. From there it's as steady as can be without blowing up. Of course some efforts above race-pace will be thrown in there based on terrain, but if I can nail a very high tempo pace overall then I've had a good race. I'm usually left pretty gassed for a day or two after an effort like that.
3. NUEs - Prior to my first NUE I'd only ever ridden my mountain bike 5-6 hours twice in my life, and had one 8 hour road ride a few years back. However, the starts are seemingly less intense unless someone like Jeremiah Bishop is around. Either way, my goal is to make the lead group for as long as I can. I wouldn't go until I blow up out of the lead group though, rather, I would let myself drop back as my legs tell me.
Pacing well is ultimately about your competition. Practice matching efforts, conserving energy, and build a solid base. Know who your competition is and on race day, match their pace!