When you ride for fun, riding is it's own motivation. It's not a workout. It's not exercise. There is only the intrinsic joy of being in the woods, conquering climbs and technical sections, and ripping the descents. In general, I find it's more fun to ride fast, and good endurance means you can have fun longer. So, here's my take on how to do that.
Speed on a mountain bike can be broken down into two major categories: fitness and technique. Most people put too little emphasis on technique. It's evident in races when they slow up in the tight sections, but hammer the straight-a-ways. It's also evident when I go on road rides and eat the dust of people I could beat on the trails. I think most of my competition on the trail could destroy me on the road.
Technique
Acceleration takes a lot of energy, so the more speed one can carry, the easier and faster one can ride. This is why technique is important, specifically cornering. To maintain speed through a corner, one also needs to maintain traction or at least slide predictably and as little as possible. At high speeds, traction depends less on tread pattern and more on line choice. The most traction is generally found along the outside edge of the turn. Traction also depends on suspension setup and weight distribution. Experimentation is key here. Loose corners need to be taken with very little weight on the handlebars to keep the front wheel from washing out. Bumpy corners need a lot of weight on the handlebars to keep the tire planted (assuming you have good suspension set up properly).
Another good technique is to look several seconds ahead rather than at what you're immediately about to run over. Make a mental note of obstacles and anticipate when you'll hit them. By looking ahead, you'll be able to brake less and maintain more speed. It takes a little practice, but I noticed an immediate speed increase when I figured this out. A word of caution: this technique can get you in trouble quick when riding on a layer of pinestraw or in tight trees.
Good posture is also important and trumps being aero - applies to both road and trail. Shoulders down, arms slightly bent, spine elongated; expect to use lots of core strength. Correct handlebar height and width is important; I'm fairly short and still ride 720mm bars level with the seat. Rarely is a slammed stem a good idea. Good posture will let you take advantage of your full lung capacity and help you maintain control.
Fitness
Fitness gets covered a lot, and there are lots of theories on how to improve. It basically boils down to strength and lactate threshold. Strength can be built with intervals (not fun), single speeds (lots of fun), and driving rickshaws (fun and profitable). Lactate threshold determines what power output one can sustain. For a newer rider, ten minute intervals are good, otherwise, racing and race-paced rides are good ways to bump up this speed limit. I find that LT training is best done on solo road rides.
I like to find that race-pace sweet spot and hover around that level of power output. I may go above threshold when riding uphill but not so much that I blow up. Then dip slightly below threshold on the downhills but only enough to recover. Learning to recover while maintaining speed (or even accelerating) is another key point. I find it's best to shift positions and focus on engaging different muscle groups. For instance I'll sit forward and engage my quads on the climbs, then shift back and focus on pulling up on the pedals on the descents.
Efficiency
When you put technique and fitness together, you get efficiency. A constant power output is much more efficient than on and off hard pedalling. Unfortunately, XC racing is largely on/off, but maintaining speed through the corners and momentum up and over the hills can increase efficiency. This is a major concern even in a two hour race and more so for longer races. Maintaining flow is usually faster than cycles of hard pedalling and hard braking.
Structure (or lack of)
I understand that some people are motivated by numerical improvements and thrive in a structured routine with scheduled intensity levels, but I've never been one of those people. Sure it's cool to see mileage and average speed, but beyond that I get bored. Power meters, heart rate monitors, cadence sensors, and even cycling computers are all unnecessary. I have one simple computer on my road bike for keeping up with mileage and time, but that's it. I prefer to ride by feel. Even in the TNGA I didn't track mileage. I have more fun just riding.
Structure may be unnecessary, but variation is good: short fast rides, long slow rides, short slow rides, long fast rides... Ride how you feel. Ride for as long as you want. There are days when you'll need to apply Rule #5: HTFU. And then there are days when knowing your limit is key. Rest days are good. Sometimes rest weeks are even necessary after a hard month of riding.
In conclusion, ride a lot, ride fast, ride slow, experiment with weight distribution and bike setup, practice the technical sections, and if you find yourself crashing too much, you're not riding fast enough.
