Everything today is all about watts. Sure fitness and improving it are definitely high on every racers list of thing to do but don't neglect working on the skills.
Being smooth and finding "free" speed should also be on the agenda.
I am fortunate that I grew up on a BMX bike. I raced them for 6 years and made my way up to a Single A Pro before I stopped. I also spent plenty of time on quarter and half pipes as well as skate parks. All things that contributed to the way I ride my mtn. bike today. Racing BMX taught me how to speedjump or pre jump obstacles. It also got me comfortable drifting and sliding my bike. BMX tracks tend to get pretty dry and hard packed so you are gonna slide around a bit. Riding ramps taught me how to pump the transition which helps build speed and enables you to get air out of the ramp. I was able to go head high out of a half pipe and could go fence high out of the Full Pipe bowl at the Upland Skatepark.
I realize that if you didn't grow up doing this stuff it is going to be kinda hard to take up BMX or ramp riding as an adult but that doesn't mean you shouldn't try to develop these skills. I will still go out once a week or so for a mtn. bike ride where I work on flowing lines and keeping my momentum at a maximum without having to pedal to do so. Perfect time to work on this is when you are riding with a slower rider. I will try to pedal as little as possible and still keep up, or if I am up front I will get a few quick pedal strokes in and then see if I can coast away from them by staying off the brakes and pumping and pre jumping transitions.
I often hear comments about how I always seem to be in a bigger gear compared to others, I don't think it is due to me just simply being a masher but more due to my style. I can use a bigger gear because I don't let corners or obstacles slow me as much so I am maintaining my speed rather than having to re accelerate back up to speed.
Just like like fitness, with work your skills will improve as well. Also just like fitness some people are just better at it than others. I went for a ride once with a 15 year old punk kid by the name of Mitch and I could tell right away he would be a good mtn. bike racer. He just had good flow, he wasn't super fit but you could tell it came naturally. If it doesn't come easy for you that doesn't mean you shouldn't work on it, most likely if your not it is whats holding you back more than figuring out a way to produce 20 more watts.
Oh yeah that punk kid is now a Pro Mtn. bike rider, guess I was right about him.