For testing, do you mean by HR or pace? For HR, dont’ worry about it. If your thresholds are close enough that you’re
stuck blessed to be doing most of your volume in Zone 1, then they’re unlikely to change much.
But testing pace could be useful if you have a prior reference. But if you don’t, then don’t worry about it.
I would stick with your plan of threshold work and Zone 1. That will sharpen your ME work and make it race-specific.
Most importantly, don’t get greedy or try and get fancy. If you feel like you’re peaking in a workout and could do more intervals than are planned, DON’T. You want to peak for the race, not the workout.
To paraphrase Canova, in the last phase, there isn’t much you can do to make someone faster, but there’s a lot you can do to make them slower…
Good luck with your race!