I'm gonna agree with Bryan, and disagree with the poster above.
So yes, get a good, reliable, read on your zones. Lab testing is the gold standard. I believe that with some persistence and care, it can be done in the field as well, though this is tough.
As for the previous post, I disagree on many scores. Unless you are running a reasonable amount
(25+ mpw
), don't do speedwork apart from, perhaps, the occasional pick-up or strides or 'I feel good let's run this last mile a little harder'. You absolutely WILL get faster by just running more.
As for your actual question: Yes, slow down. Once you know your zones reliably, if your plan or intention
(for whatever well-thought-out reason you may have!
) is to run in Z2, then yes, slow down however much it takes to be in Z2.
The point of training in different zones is that you gin specific benefits from training in specific zones. If you avoid training Z2 because it is 'too slow' then you will lose the benefit of training in Z2.
(In my book, one of the main benefits of Z2 is that it is not particularly taxing, hence allowing you to get in more volume more consistently. But it also provides specific physiological benefits.
)
Once you are up to 25mpw consistently and you feel strong, add in some speedwork.
My 2c.