I just did one myself yesterday. I did 35 minutes. Once I felt out my steady state and confirmed my FTP, I ended the interval. I could have gone to 45 minute or longer but there is really no need. My legs feel like they got a good workout.
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The lactate thing is physiological reasoning and hand waving only, nothing definite, so Iâm very open to it not being correct. But itâs not about how high the lactate is, itâs the duration of having to deal with the discomfort that would be different.
And to address your last part. Again, Iâd do 4x4 VO2max where HR > LTHR and your gasping the entire time, over 2x20min @ Alpe du Zwift pace any day. So itâs not an issue of great anaerobic capacity and FTP set too high 
YMMV. But I can only speak from my personal experiences.
Preference is one thing but comments like this started this tangent:
I find VO2 Max work relatively easy, but long (8min +) threshold sessions very difficult, both physically and mentally.
Eight minutes at threshold should not be difficult under any circumstance other than the rider doesnât know what their threshold is.
And VO2max work shouldnât feel easy either, not even relatively easy, which means one isnât going max.
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I think weâre going in circles 
Probably the take away point is that people mean different things when they talk about a workout being difficult, and when giving advice itâs useful to clarify what they mean.
Mental and physical difficulty are different beasts. Mental difficulty is subjective and can vary between people. When people just refer to difficulty in general it can be unclear if theyâre talking about mental, physical, or some combination of the both.
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They said ârelatively easyâ as in âI find VO2 workouts relatively easy when compared to threshold workoutsâ. Thats different from saying the VO2 work was easy.
And again, I think this really depends on how youâre defining VO2 work. Personally, I think 4x4 at 105% IS easy compared to 4x8 @100%, and I donât do that kind of workout to improve my VO2 Max, but by definition it IS a âVO2 (Zone 5)â workout.
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I totally got that people are different and they rather do a painful short interval instead of a longer one, got it. Who am I to judge or say otherwise on oneâs pain?
But again, if you somehow find VO2 easy, youâre not doing it right! And I have to agree with @AJS914 that 8min at around threshold should not be excruciating.
Iâll reinforce my suggestion, do 5x5 WITHOUT POWER/HR, all out! Go really deep, and report back.
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This is EXACTLY why I keep beating on âdifferent people are talking about different VO2âsâ
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Those seems to me the onesTim Cusick suggests in the WKO5 webinars as a maintenance VO2. 5x3 min âtouching in VO2â. Not even this, itâs just a âwake upâ every 10 days. Which is way different than VO2 as improvement.
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So do I
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Motivated by this thread and a rainy Saturday, I decided to do a 2x20 @ 100% FTP to see how things go.
I havenât really tested my FTP lately (laziness and lack of need). Intervals has it set to 293w, which seems to be right.
2 sets average 290 (just because I like rounded numbers) each. And yes, itâs totally doable. Hard, but doable. My HR barely entered the threshold HR. My fan solution is terrible, so I guess part of the HR increase at the end of the second interval was due to the heating. The very last 2 min of the second intervals were a bit challenging, rest was pretty much the definition of âhard but doableâ. 0/10 Iâd say 7.
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Nice!
This sounds just about perfect to me. Sounds like time to step up to 2x22â or something next time
It was my first indoor season in a long time. As the season is coming to a wrap, and I have to take care of my daughter, so indoors will be. Not a huge fan, though.
You gave me an interesting idea, I might go for a 3x15 next Saturday.
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Haha yeah it definitely takes time to get used to the inside sessions.
3x15 is good too. The goal should just be to increase your interval time by like 2-5 minutes each time. If you canât then itâs a good indication that youâre probably a bit fatigued from your previous sessions. Try targeting something like 60-70min. Itâs pretty cool doing a power for 60min that just a handful of weeks ago you could only do for 40.
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3x15 are awesome. A 4-5x10â is great too. When I have time I like to split them up evenly over a longer ride, say 4-5 hours, and I find it builds a lot of fatigue resistance. A 10-15â effort at hour four is very intense.
The race this past weekend was ~3 hours. NP was ~290 watts for the first 90 mins, and ~260 watts the second 90 mins, with 310 watts NP the final 15 mins. My FTP is only ~315-320 (best 20â power this year is 340w) so being able to do right at threshold for the final 15â of a hard race and throw some watts bombs along the way was a big deciding factor against folks that were as fast or faster than me that just had more fatigue at the end.
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You guys are giving me some cool ideas!
Iâll have my daughter week on week off from now on as part of my divorce. So in the week on, regardless of the wheater, Iâll have to ride indoors.
Thanks
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These are great. Doing like 15â on the hour every hour over a 4-5 hour ride is tough but as long as you build up to it and eat enough itâs surprisingly achievable
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Fueling is key for sure, and yet another reason to do them this way. Experience is the best teacher, and doing 4-5 hour rides if nothing else will give you that learning experience you need to do better the next time.
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