I’ll take a stab at this because I happened to be reading the section on interval training in the new book just now.
I’m not sure what workout or guidelines you’re referring to. I’ve been following the 24 week plan and the Z3 muscular endurance workouts don’t start until the specific period (and even then I’ve read here the vertical beast mode workout is supposed to be Z1 or Z2 in your heartrate but feel like Z3 in your legs).
That being said – that sounds like a lot of time in Z3 to me too. Reading the new book, (p.176 and the review on p.266) Aerobic Endurance/Z3 should be either continuous Z3 tempo for 20-40 minutes or intervals of 8-15 minutes for a total work volume of 40-60 minutes. So… based on that, I’d say not only are you OK breaking it up, it seems like you should.