MTB or Xterra training plans

There are no 'Xterra specific' training plans.  You should be fine using the sprint or Oly plans.  Though make sure to get at least one training session per week for each the bike and run on the terrain you will be racing on.  Xterra courses can be pretty rough.  Though if you are a complete beginner to exercise or the bike or run...starting out for a month on flatter, smoother bike and run courses would be better for your body to first get used to the training, THEN you can add in course specific training a few months later.