Ryan,
Best answer I can give you is here:
Intensity Guidelines for 80/20 Running
Keeping Your Zones Current
As your fitness level changes, you will need to adjust your zones to keep them current. The easiest way to do this is to perform a Talk Test during a Foundation Run or Recovery Run whenever you feel that you have “outgrown” your current zones.
Races, of course, are natural opportunities to update your pace zones. For example, if you complete a 5K race during Week 6 of a 12-week 10K training plan, use your 5K time to update your pace zones.
Many athletes like to update their zones on a regular schedule. If you wish to do so, choose a preferred testing method and repeat it during every second recovery week beginning with the first recovery week of your plan. Note that recovery weeks fall every third week in all of our run plans. Thus, if you elect to perform scheduled testing, you will test in Week 3, Week 9, Week 12, etc. Your zone tests will be least disruptive to the overall training process if you do it in place of the most challenging workout of the relevant week that features efforts in Zone 3 or higher. For example, the most challenging workout featuring efforts in Zone 3 or higher in Week 9 of our Level 1 Half-Marathon Plan is a Hill Repetitions Run (RHR37). This would be the best run to replace with a test.
Also note that if you are an intermediate- or advanced-level runner and you use heart rate as your primary intensity metric, you probably don’t need to update your zones very often. This is because LTHR doesn’t change a lot with changes in fitness once you’re past the beginner (or starting-over) phase. What you will find as you gain fitness is that you run faster and faster at the same heart rates. Indeed, one simple way to update your pace or power zones is to do a test where you run at your current known LTHR and identify the corresponding pace/power, then plug this number into the appropriate calculator. For example, if you know that your LTHR is consistently stable at 160 BPM but you notice that you’re running faster at any given HR lately, do a run where you lock into a heart rate of 160 BPM and note the corresponding pace/power. Say your pace is 7:07/mile at this HR. This, then, is your approximate Threshold Pace. It’s best to do this particular test within the context of a scheduled run that targets Zone 3.
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