Mel,
I had an awesome mtb race at the Winter Park Crankworkz XC this weekend (same altitude as CB, not sure why I felt good here and not there) but I noticed that my weakest part of the race was the flatish and moderate climbing roads. I didn't feel like I had the legs to power through those sections so I just rode them at a good pace.

What kind of workouts help to improve on those parts of the race?

I think that just training specifically for that will help. That is, if you normally do hill repeats on something steeper, maybe find less of a grade and work on that. It is a different kind of power turning over a big gear on flat stuff rather than pushing a little gear uphill and it is not uncommon to struggle with one and excel at the other. You could look to work on your weakness for a few weeks if you need to ride better on the flats. Keep in mind what kind of race you have coming up, however. I would say that all of the Championships events in XTERRA are straight uphill from here on so making sure you are the strongest climber possible will be important.