I agree - go for it! In the beginner class, you won't be the only one walking sections of the course. If there is someone who comes up behind you riding, try to get out of their way (they have the right of way), but you won't hose anyone up. I have found mountain bike racers to be much more considerate and understanding than road racers (I do both), so I don't think you will have an issue.

Have fun, and good luck!

SheFly