Endurance Exercise

I’ve quoted selectively from http://www.bodyrecomposition.com/training/methods-of-endurance-training-part-5-interval-training-part-2.html
which is quite long.

Adaptation to both varieties of anaerobic intervals below seems to reach diminishing returns after just 2-3 weeks (6 sessions).

Training anaerobic power is pretty much what you’d imagine.  Generally work bouts lasting 30-60 seconds are used with fairly long rest intervals.  For a one minute maximum effort, a rest interval of 4-5 minutes of easy recovery might be taken.  The idea, essentially is to generate a lot of power (and a lot of waste products, some have even called this lactate production training) during the work bout and then give the body a chance to clear it during recovery.  That’s then repeated a number of times (perhaps 8-10 in total depending).

Anaerobic capacity training, as noted above, is more along the lines of repeated longer repeated efforts (60-90 seconds) with rest intervals of perhaps 1:1 (so 60 seconds rest for a 60 second interval, 90 seconds for a 90 second interval).  Intensity has to be lower here and the goal is to maintain power output against increasing fatigue (some used to call this lactate tolerance training).

this energy system is not massively trainable.  Certainly buffering capacity can be improved (and science continues to understand the adaptations involved) but overall improving the power or capacity of this pathway is relatively limited; you put in a lot of very hard, very draining work for not a lot of payback.

the adaptations to this type of training occur quickly but then stop just as rapidly.  Studies find that the improvements to this type of work typically stop after about 3 weeks.  In one study of cyclists, 12 HIIT workouts over 6 weeks generated no better improvements in performance than was seen after the first 6 workouts over 3 weeks.
in Effects of Moderate-Intensity Endurance and High-Intensity Intermittent Training on Anaerobic Capacity and VO2 Max a similar pattern was seen: the major effects occurred in the first 3 weeks with far smaller effects occurring in the last three.
 the 2000 German track cycling team, preparing for the 4km (an event lasting about 4 minutes); they performed miniscule amounts of anaerobic training and did so for about 8 days prior to the event.  They had found that that was all that was required to max out the anaerobic pathways.
the high acid production caused by these types of anaerobic intervals may damage mitochondria and overall endurance especially if it’s not balanced by sufficient low intensity work (Lydiard, for example, was adamant that anaerobic work never be performed while building the aerobic engine).  This was an older idea that has seen recent scientific validation: in a recent study, individuals who consumed bicarbonate prior to training obtained better aerobic adaptations, presumably by buffering acidosis
Aerobic training for VO2 max also reaches diminishing returns in just 3-8 weeks (8 weeks starting from a very weak base).
 Interval training, specifically the kind that drives athletes to achieve VO2 max primarily affects heart function.  Certainly there are going to be peripheral effects in muscle but the main effect is in heart.
VO2 max can be sustained for 5-8 minutes at absolute maximum, generally VO2 max intervals are performed for anywhere from 2-5 minutes (with 3 minutes being a fairly standard duration).  It will generally take about one minute for the body to achieve Vo2 max such that anywhere from 1-4 minutes is performed at VO2 max.  With multiple sets, a rather large amount of training at VO2 max levels can be achieved without having to go to complete exhaustion.
Rest between aerobic intervals should be 50-100% of the duration of an interval.

Steady state aerobic endurance requires and develops an endurance adaptation in muscles
 interval training improved Vo2 max far more than long-duration training, it had little to no impact on citrate synthase levels [evidence of aerobic endurance adaptation in muscle]. In contrast, steady state training primarily improved citrate synthase levels with little impact on VO2 max.
I’m sure that starting from very low fitness, you could see improvements to both with some intermediate duration.  But in already fit people, you really mostly only build one thing at a time (and sometimes that work can actually actively degrade other areas).

Steady state means, of course, that you could keep going for a long time (an hour).  If you want to adapt faster, you’d work near the threshold of the most intensity you could maintain for that time.  It’s probably good to do more sets (throughout the week or back to back) of shorter duration (10-20 min) rather than actually working that hard for an hour.  I don’t even want to determine what my limit is for an hour; I’d just work really hard for 10-20 min and then reduce the intensity by a little bit from that benchmark as an estimate of what I could do for an hour.

Goal Intensity Work Interval Rest Interval
Neuromuscular Power Maximum 6-15 seconds Complete (2-5 minutes)
Anaerobic Power Just Below Maximum 30-45 seconds 2-5 minutes
Anaerobic Capacity High 60-90 seconds 60-90 seconds
Aerobic Power Above maximum steady state 2-5 minutes 2-5 minutes
Aerobic Capacity At/near maximum steady state 8-20 minutes

4-10 minutes