@AlexM370, great question on that. @bgkeen has excellent feedback. The one other piece I will add is that you can always adjust the recovery period to tune into the appropriate % for what you are trying to accomplish. So you can take a 4x8 and treat it a bit like Trevor’s famous 5x5 workout where there are very low rest periods (1 minute) and this allows you to spend a lot of time just below threshold to reduce the anaerobic contribution to the start of intervals and accumulate a lot of time at just below threshold. You can also extend those rest periods and focus more on a hard-start approach or working at >100% threshold, knowing that there are other benefits to gain by going to that level of intensity, but at the expense of taking more recovery from a session like that.
One of the blocks I’ll do is 2x per week 4-5 intervals of 4-5 minutes zone 5 with a hard start approach. I’ll knock out 6 sessions total, usually within ~3 weeks time, and then rest hard. There is absolutely no other quality work I can accomplish at that time, but when I come out of it I feel good and ready to race. Threshold bumps up, lactate becomes more tolerable, and time to peak power takes a step in the right direction.
I am in agreement with @bgkeen where he talks about being too fried to have any good work the next few days. Those can be very taxing sessions, and if we think about 32 minutes at 106% or greater, that may take a while before we have good legs back to go hard again, versus something a little more sustainable sub-threshold where we can accumulate more time. So yes, very individualized, and also specific to what you are looking to accomplish at that time. For testing, there will be those days where you feel really good and can knock it out. Like the day to do your “max” testing in the gym.