5 or 10K?? Thanks
In general, not knowing anything about how much you run or your strengths and weaknesses, I would prefer 5k. You want to do the distance that is going to give you the best chance of hitting a good test. The one that brings you closest to your ability level. If you are going to have a bad day, a longer distance like 10k will likely magnify it. Plus I find with the people we test that the problem is not usually a lack of speed but of endurance through not doing enough miles. The longer the race distance, the farther they tend to perform from their potential.
Besides, it easier to come back soon and try again after a bad 5k than it is after a hard 10k.
That’s just in general though. If you are someone who naturally tends to perform better at longer distance races, perhaps the 10k would be better. And in the interests of full disclosure, I have an almost exclusively running focus.
In previous years, I have found that the zones often would not carry over to longer distances. I could hit the times in the shorter distances 5K,10K, usually 1/2 M, but could not carry pace over to full marathon.
10k to 15k
it’s not a distance that makes it accurate it’s the duration
.
I find that myself. I think part of it is the distances people are suited for. I’m a much better 800 meter runner than marathoner, no matter how many miles per week I run. But when I do run high mileage, the performance gap closes down quite a bit at the longer distances
When you say “hit the times” are you referring to the training paces or to the race times that the predictors say you should be able to run? If you are unable to hit the training paces, that’s not good. But being unable to hit longer race performances is not unusual.
I’ll answer for Joe here since I have the same problem. The answer is more long, slow miles. The potential is there (performance in shorter distances) but the run fitness is not (falling apart as distance increases).
Run more.
it’s not a distance that makes it accurate it’s the duration
Very nicely worded
.
my coach would usually use a 5k running all out with maximum hr coming at the end
.
it’s not a distance that makes it accurate it’s the duration
Bingo.
There is a pretty strong correlation between the pace you can hold in a one hour ( 60 min.) running time trial and what your Lactate Threshold pace is. Obviously this is not the best test for newbies, but for more experienced folk it’s a good way of finding out what that pace is and what it feels like to sustain that sort of pace for that length of time.
For some, they will find this in either 10 mile or 15K running road races. Problem is, there are not that many of these - there should be more! 
The test most people use and rely on (5K) is generally not the best to use for data gathering IMO. Lots of people use a 5k to determine results though because running a LTT 10k every couple of weeks is pretty taxing and can add up over time. Thus, many people choose an “easier” distance that they can repeat more often.
Any time you take a smaller sample data set, your end result predictions are not going to be as good. Larger sample size = better results so by doing a longer test, you are getting better data. That being said, any “bad day” affect will show itself more on a 10k than a 5k.
Bottome line is that as long as you are putting forth a good honest repeatable effort, it will be good enough to figure out LT pace and eventually your pace guidlelines.