I think you have the emphasis wrong, Fred. And it is a mistake to equate LTHR with 90% of MHR as a generalisation. LTHR needs to be tested for specifically. In my own case, my max HR is somewhere in the mid-180s. A couple of years ago it was 186, but I havent seen anything above 181 for a few months so it may have dropped a couple of beats. My LTHR, however, is 156: nearer 85% than 90% of MHR. I could not complete two 20-minute intervals at 90% of MHR.

And I'd also take issue with your underlying assumption, which seems to be that only anaerobic intervals count as real intervals. Absolutely not, in my view. I said at the outset that I wouldn't recommend anyone bothered with Z5 work at an early stage of their training, and I stick by that. 2x20 minute intervals are at threshold, one simply can't complete them much above threshold. But they are plenty hard enough. Shorter efforts will and should take one above LTHR, of course. But until one has an established base one should be very careful with them. To quote Joe Friel from his cyclists training bible,

"anaerobic endurance training is quite stressful and should not be a part of the novice cyclist's regimen. Both speed skills and endurance should be well established, with at least two years of training, before these workouts are attempted on a regular basis. The likely result of too much anaerobic endurance work too soon are burnout and overtraining. "

You did say you were feeling more vulnerable to illness and injury, right? I'd be careful. And just to be clear, anaerobic endurance intervals range from three minutes at 9/10 RPE with equal recovery time, to less than a minute at 10/10 with very short recovery. This is the stuff I need to do more of to improve my crit racing. I really wouldnt recommend it for those who are just in the process of getting fit.

Following Friel, I'd suggest an initial focus on longer efforts for muscular endurance, not anaerobic endurance. Several minutes at a time in HR Z3, building up to longer intervals (ultimately up to an hour) in Z4, nudging into Z5a. . Again to quote Friel

"the effort is much like controlled time trialling and is tremendously effective in boosting both aerobic and anaerobic fitness with little risk of overtraining"

Approaches vary, of course. But for the cyclist who is just starting to get serious about training systematically, I'd concentrate on the tempo work, and save the really fearsome intervals for later. There's no hurry, and if one wants to get really serious later, the very intense stuff will be better tolerated, and therefore produce better results with less risk, if one has spent a longer time establishing one's aerobic base.