I think you need to evaluate what is slowing you down...

Is it the climbs that are killing you? Is it the distance? If so, maybe more road training or spinervals would be good.

What exactly is slowing you down? When you ride with people who are similar or a little better, people you will compete with, where do they gain in distance?

I think a BIG part of mountain biking is handling. Can you take the turns as fast, the downhills as fast, do you pick the best line, etc.? I know I hit my brakes more than DH in the tight switchbacks. He'll also plow between two trees that are about as wide as my handlebars, whereas I'll slow down a little, he will take more speed on the downhills. If you are slowing down for these reasons, not cardio, I would say hit the trails more and work on your handling.