Most folks aren't really familiar with what training for competitive cycling (or structured training in general) looks like.
There are large, month-scale blocks where you're simply building aerobic capacity by cycling at low-ish intensity. Heart rate is somewhere between 115 and 145 for me personally during these efforts. 15 hour (or more!) weeks are not uncommon.
Nils van der Poel was doing something like 6-hour blocks of zone 2 during his base season as an Olympic speed skater.
When outdoor riding is available, it's preferable, but indoor trainer sessions are basically always more time efficient for a given training load. You make consistent power, there aren't street lights or flat tires. If you run out of carb mix you go to the kitchen and fetch more. In winter, it's the only reasonable option. Going for a ride in -15C and salt slush is fine for a commute (crazy, I know), but doing it for 15 hours a week, probably in the dark, is not reasonable. Indoor is the only practical option to get this training load in winter.
Most people just watch TV or use Zwift (social riding simulator) during these blocks. I'm not much of a TV guy, and to be honest, I find a good technical problem to be great distraction from the drudgery of Zone 2.
Note for OP: I've actually considered using a split keyboard in a very similar way to this. I run aero bars on my bikes for ultra-distance events, and I have them on the trainer right now. At some point I would like to mount a split keyboard to the aero extensions themselves - this would take all the load off the wrists. It is also possible to use the aero bar armrests without the extensions and to be able to type this way. If I end up prototyping this I will shoot you an email.