I did 2 a day for the better part of 2 years. I did anywhere from 8-15hr/wk. About 30-90 min per session during the week. Weekends were anywhere from 60min to 3+hr per day, can be 1 a day or 2 a day, but rarely.
To be clear, I didn’t use TR plans.
I would normally have high and low Z2 work, low Z3 work, and upper Z4 work upwards of 40 minute intervals and as low as 8min, varying morning and night. VO2 sessions every other week or so. Regen work would be high Z1/low Z2. endurance Z2 work around 70%, and the rest are intervals of some form.
2 weeks increasing TSS, 1 week regen at lower TSS, but not a huge TSS swing, while progressing overall TSS ramp by week. Ex for illustration purposes: week 1 may be 550 TSS, week 2 625TSS, week 3 400TSS, week 4 610TSS, week 5 675TSS, etc. Ramp rate was moderate, not sharp and not flat. I have to say, I felt strongest during this period. I also did exercise almost 7 days a week. Every now and then would have a rest day, but it was less frequent and irregular.
You need to manage your load and strain accordingly, and progressively overload. I didn’t do a whole lot of “SS” work as defined by TR. I would say more tempo than SS, but would hit the SS part of Z4 now and then, and would hit Z4 (95-105%) intervals more regularly. Some would be 1x intervals, some 2x, and 3x or more if longer. A mix of short and long duration, with intensities to mark hard, and moderate days. If I’m feeling a bit more tired, I might hit a 2x10 instead of a 1x20, for example. Rest between intervals are kept short, maybe 1-2 min.
What I liked about it is that you get some recovery throughout the day, and when you get on it, it’s business and done. Though with all the warmup and warm down, and some regen work. TiZ for Z1 was really high.
Generally, I wouldn’t advise it, unless you have an idea of how you’re going to track. off-the-shelf substitutions, IMO, won’t work well. You have to manage micro-cycles, and really gauge fatigue honestly and not try to hammer SS every session. There will be weeks where you’re just tired, but with a 2on/1off, it’s not extensively so. Tracking time in zone for Z3, Z4, and Z5/6.
That said, doing anything is better than nothing. FWIW, I was also strongest during this period. Timewise, it’s longer, in total. It’s counter-intuitive but you have to shower, and prep, etc. which adds up. But, it fit my schedule better than a long block per day and you miss out on longer work except for weekends.