I would like to preface this with the fact that this is the run training that is working for me and may not be the right thing for everyone.
Given that I have a history of injury from too much running, I don't run very much. That being said, I think a lot of people run too much and that is why they get injured
So, because the race is usually won on the bike, you need to ride a lot. Then you need to be able to run STRONG, not necessarily FAST. You cannot get a rhythm in Xterra runs so the important thing is to turn your legs over the fast you can while they get more and more tired.
I do four runs a week. I run sunday mornings long, about 90 minutes. I do three other runs a week, two off the bike, one with some pickups, one just shuffling while I am blown and the third as a long tempo run or a treadmill run.
I think running longer and harder is more important than short speed workouts which are good for speed on the road but don't prepare you to run strong when you are tired.
However, even this is a lot of running for me. Before this season (ie 2006) I ran two days a week and won the world championships, twice. Running was still my weakness, however. If you do not have a lot of _base_ for running, or you have limited time, do all your running as runs off the bike and you will be amazed at how much better you run in triathlons.